I wouldn't worry too much about it. Just check in at exactly 24 hours before the flight for the best seat selection.

Even if you can't get seats together, UA staff will not allow children that young to be by themselves. The worst that will happen is that you (+1 child) will be separated from your wife (+1 child). Or one adult will have to sit with 2 children, and the other adult will be elsewhere. That might be a good thing: one adult gets a break from child care for half the flight, then you switch.

Good luck!