I have twin girls now 6. They went through a phase like this when they went into their toddler beds. It is hard for little ones to take a change like this in stride. It is so freeing to go from a bed with bars they can't get out of to a fun low bed they can hop in and out and get in bed with each other etc. We didn't have the problem at night, but we did during nap time. What worked great was sitting in their room in a chair and I'd have my quiet time with God or just look right at them or read a book and every time a head popped up or anything that was not okay I'd just say put your head back down, or time for nap or something like that. It took one week. 1st time it took an hour and it went down from there until by the end of the week it wasn't a problem. Before the nap tell them they can play when they get up, but now it is time to take a quick nap so you have set your expectations. Worse case, they might need a little pop on the bare leg to show them you mean business if they won't put their head down when you ask them too. Even though some Moms might consider this a waste of time for a week, it really is enjoyable. The room is quiet and kind of dark, they look so peaceful laying there. Good luck! This too shall pass!