From my experience for these things you either pay up, or it is a gamble, especially if you want the "good" seats (like first row, or emergency exit).
Check for paid options...
Delta sells those as "Preferred Seats"; so you need to pay to get those - and if it saves you pain, you should (obviously you can't take any designated emergency exit row with a toddler). Don't wait if they are available for free during checkin: At Delta, frequent flyers can reserve those in advance, without cost, and the they are gone.
You can also check if there are any paid upgrade options within your financial reach.
...or ask at every step of the way
Failing that, you can try to check in online as early as possible, and see what seats are available - get the best ones you can get. If you're not happy, ask again at the bag drop counter, and ask the check-in staff before check-in starts. They are often not able to help, but if you ask nicely they'll at least try if they can shuffle things around.
If all seats are already taken, little can be done before check-in. In this case your best bet is to ask the cabin crew for assistance. They can't (and won't) evict other passengers from their assigned seat, but if you have a valid reason they'll usually see if they can switch you around - and having the crew "officially" ask works better than to approach people directly. The cabin crew may also offer additional advice or help to deal with the motion sickness.
If you don't insist on first row, but are okay with a window row over the wing (where there is the least motion), chances should be good that they'll find a solution.
(Also, if you get separate seats, getting the other person to swap shouldn't be a problem. Most people are more than happy to switch away from a toddler on a long-haul flight, especially one that is going to be sick...).