They won't just fall out if theyve never been in a pool. They will try and step up on the edge, and if they can not, they can't get out. Put a step stone in the kiddy pool, that way if puts them up higher, make it easier to get out. My girls, I was told, when they first started using the pool, they had problems getting out. Now they can get in and out no problem. Have to learn.
Hope they are warmed up good now. Good luck.
ETA< sorry, just saw that you planned on doing that.