I'm traveling with wife, 6 yr old and 2 yr old son. Wife is asking me where is the best seat (Economy) for our family of four, comfortable for the kids. 15 hour straight flight.
After checking the seat layout provided in @pnuts's answer...
No good seats to fit your exact needs, I was thinking of the bulkhead seats, but it seems in this particular airplane for this particular airline the bulkhead seats have "restricted leg room", which is not the case in many other airlines, as they add few extra inches to the leg room in bulkhead seats to compensate for the fixed arm rests and for the in-seat tray tables. In addition to that, in this particular airplane, the only bulkhead seats which are not emergency exits are the one at the beginning of economy class, and they are a mile away from lavatories, so bulkhead seats are a big No.
More bad news
You don't want to take seats on the side (A window seat and the two next to it), and then take one more seat on the middle to get your four seats, this is because seats on the sides are not aligned with the seats in the middle.
The best of the bad options
Take any four seats in the middle, at least you will be next to each other, kids in the middle and parents on the aisle seats. Choose a row next to lavatories, not too close to avoid noises, choose ones next to galleys.
B777-300 is a nice aircraft in general. Sorry, can't find any other good news :)
It is imperative that you
BOOK EARLY TO GET FOUR SEATS TOGETHER.
Put all other considerations aside. There are no "good" seats in that plane, no difference.
As soon as you see this, SELECT YOUR SEATS so that you get four together.
It is very likely that you will get caught out by this new crap where the airlines will magically require you to pay a few more dollars, at the time of seat selection, to get four seats together: as soon as you see this, go select your seats!
Again, this is a 3-4-3 plane so it's messed-up if you don't get four together, for a family of four. Go for it!
Some months ago we (2 adults and 2 kids of 2 and 6 y.o.) had an 11 hours overnight flight on a 777-300ER from another airline. As there were no rows of 4 we took 2 times 2 seats: 52D, 52E, 53D and 53E. My husband and me were sitting next to the aisles.
Despite the negative advice from SeatGuru, this choice turned out to be a good one. The plane was not full so that one of the G seats on were empty and we had thus some extra space. Moreover, being close to the galley was good as the stewardesses were really nice and helpful. And the proximity of a lavatory was also a good point. We would straight away pick these seats again.
The Philippine Airlines 777-300ER is configured differently. There are indeed rows of 4 int eh economy class. However, I would pick 71A, 71C, 72A and 72C or 71H, 71K, 72H and 72K. Or maybe the same seats but on rows 70 and 71.
And I can confirm that the 777 is a nice plane.