DS who is 3 does this too and has for months- he sleeps from 12:30-1:30 until around 4 - if it is after 4 we have to wake him and that is usually when he is grumpy Makes it really hard because we can't let him sleep longer than that or his night sleep is stuffed up but it really upsets him. I wake him so gently, just stroking his leg, hair etc. and sometimes I just open the door and make more noise than usual but he will still crack it because he wants to sleep more! Sorry I am no help - I just give him loads of cuddles til he gets over it plus I distract him from it all if he is really bad by putting an episode of Thomas on for him. Good luck and I hope they grow out of it quickly.