While I could see a gate agent being willing to shift people around to make sure your children aren't seated alone, I would not bet on them working hard to get all 5 of you seated together. If at this point you can get some pairs together anywhere, then I would go ahead and take those. Or if you absolutely must all 5 sit together, if there is currently availability in E+, pay for it now.
I don't think it would be a good idea to wait until check in to assign any seats.