I recently flew with Thomson and I was separated from my 3 year old child on the flight home. He was sitting a row behind and across the aisle from myself. I asked several times if we could be moved closer together but the air hostess was not interested. My question is how can a 3 year old be expected to sit alone unaided next to a complete stranger? I couldn't observe him all the time from where I was seated and once I turned around he was half out of his seat with his seat belt around his neck. My other concern is how do I know who he's sat next to and that he's safe? He could have been seated next to a pedophile for all I know and as it was a night flight a lot of people were sleeping and the lights were dimmed. This raises serious safeguarding issues. Aren't there any laws to protect young children when they fly?
Airlines today charge for everything, including choosing your seat. If it's important, and a matter of safety, that your seat be chosen in advance, such as seating two people together, then you need to spend whatever amount the airline charges to ensure that. It's not a practical strategy to assume the charged-for service will be provided to you for free because it's a safety-related matter.
(This is all the more important if opinions vary on whether it's a safety issue or not; as you've seen on this question many people will deny that it's dangerous for a three year old to sit away from a parent. My point is only that asserting you have an important safety reason for receiving (for free) something that is normally charged for will not work, and that the correct strategy is to pay for it.)
That said, once you find yourself in that position, to increase the chance that random strangers will give up something they've paid for to solve your problem, I recommend being as polite as you can, acknowledging you're asking for a favour, and offering an incentive. For example if you are in an aisle seat and your child is in a middle seat, it's unlikely anyone will want to switch into that middle seat so that you can sit together. Offering your aisle seat to the window and aisle seats next to your child might be seen as an improvement for those people. Approaching a window/middle combination and offering each of them the aisle seat you have and the aisle seat your child has might look good to them. Put yourself in their shoes and ask what it might take to get your help.
Do not make it the cabin crew's (the name air hostess is from a long time ago and not in use today) job or problem to solve it for you. Under the terms that airlines use today, it simply isn't. Don't demand your "rights"; you don't have any. Don't accuse the people you are trying to trade with of being pedophiles who planned a trip to France or wherever with the hope that they would get an unattended toddler next to them. That's not going to leave people wanting to work something out with you.
The Thomson Select A Seat page implies that you can purchase seats together for extra money. Although you may be re-assigned and, interestingly, their definition of 'together' is:
Seats together may be across an aisle or behind each other.
So that may not solve the problem. That page say Thomson follows the Civil Aviation Authority guidelines, from the CAA seating page
The seating of children close by their parents or guardians should be the aim of airline seat allocation procedures for family groups and large parties of children.
Young children and infants who are accompanied by adults, should ideally be seated in the same seat row as the adult. Children and accompanying adults should not be separated by more than one aisle. Where this is not possible, children should be separated by no more than one seat row from accompanying adults. This is because the speed of an emergency evacuation may be affected by adults trying to reach their children.
So it appears that there's nothing, legally speaking, that Thomson did wrong. (Arguably you could read that in such a way that what Thomson did was pushing the line, but I don't think it was over the line).
In your specific case my first question would be why you didn't switch seats with your child? At least then he'd be visible to you and generally nobody would have a problem with that.
I'm surprised nobody offered to switch seats, but given this is a holiday flight I guess a lot of people were together in groups and you'd have been splitting up some other group. Since it sounds like a full flight the assistants may just have been too busy to try and help.
To avoid this in the future I would look at selecting seats in advance (and paying for it), this should be possible. Otherwise when you check-in double check your seat assignments and talk to the person at the check-in counter. It's much more likely they'll be able to arrange something there before everyone is aboard the plane and seated.
Above all, be polite. As I said they were not breaking any laws or guidelines, therefore you have to rely on them to do you a favor. They'll well within their rights to do nothing and if you come across as angry, impolite or demanding they'll simply opt not to help because they don't have to.
The pedophile comment makes me think you may tend to worry a little much. Honestly, a) most people are not pedophiles and b) nobody is going to attack a kid on a full plane. (And most attacks are don't follow the common 'stranger-danger' trope, but rather are committed by people that the kids already know).
I'm not sure we can be of much more help. If you want to take it further I would suggest writing a nice, reasoned letter to Thomson asking for clarification. You might get a voucher or better advice for the future. You could also take the case to social media (here's their Facebook page) and try and get a response from them there.
Next time, maybe swap seats with your son? Then at least you don't have to crane your neck to check on him - he will always be in your view.
As for the danger of pedophilia: the preponderance of pedophile crimes involve a trusted adult, not a stranger. Going by statistics, your family members present a danger many times greater to your son than the stranger in the charter plane.
This is an interesting conundrum. Of course it's totally unacceptable to have three yer old sitting alone and I'm actually surprised (and sadly disappointed) that none of the other passenger would trade seats. It's atrocious that the airlines allow this.
But then again, not all seats on the plane are the same and a better seat costs more these days, especially for budget airlines. By not paying this, you expect a benefit for free that other passengers need to pay for.
So you get what you pay for: chosing a cheap ticket on a crummy carrier will give you a cheap and crummy experience. There are other airlines where you can select seats upfront for free, but they tend to charge more for the ticket. It's just a different way doing business.
So going forward, you need to factor the cost of seat selection into your travel budget. It's free on some airlines and not on others. You need to make sure you understand the rules before you book.
It would be really nice if the CAA or other regulatory would make this mandatory for children, say, six and below. This would force the airlines to be more transparent about it. So they either need to charge a "family seating fee" or do it for free if they feel the public backlash would be bad for business.
I suspect that if your child had started crying "I want to be with my Mam", you would have found much easier to change sits. I don't think we should encourage our children to act like that, and I don't like that conclusion. But there's little you can do. The other passengers should facilitate that.¹ Just because a child is quiet, it doesn't mean he is enjoying the trip, he may be completely frightened (and will probably fear at some point, while just grasping your hand would have helped enormously).
¹ Or if you are absolutely not willing to change sits, you should look after the child a bit, which is clear from your description they weren't doing. As the children grow, and are more used to flights, they may not have a problem with it. However, in this case I find your child too young for such a long flight.