On the evening after DD2 was born we realised that there was no way that we could maintain sitting with DS, DD1 and deal with the baby, so DH went into DS's room and told him if he went to sleep by himself for three nights in a row then he could go to the shop and pick anything that he wanted as a reward. Worked! He had just turned 4 at that stage though, so a bit older than your kids and we had done the prep of leaving the room to go to the toilet/do a job before returning to help him go to sleep so he was used to being alone in there. DD2 was only 2 and a bit when she started sleeping independently - we were on holiday and she was in a shared room with DS and I actually had to go to the toilet so I left them, then I could hear them happily chattering so I didn't go back in and she was fine. We kept it up after we got home. She has been my best night sleeper.