We're dealing with this, too. Nik and Nate start preschool in just over two weeks. We started working on an earlier bedtime last week. We just pushed bedtime back 15-30 minutes every night for a few nights and let them wake when they are ready. It has worked quite well so far, which was really surprising to me. Nik isn't particularly happy about having a bedtime, but he's doing it without too much grumbling.
Our only issue (and is was an issue before the earlier bedtime so it's nothing new) is that sometimes Nate, who just turned 3, gets super tired at around 4pm and falls asleep! Then he wakes up two hours later and is awake until well after 11 or 12. Then starts a crazy cycle for a few days of trying to get him back on track. 85% of the time he doesn't need a nap, but I don't know what to do on the days he does! I can't get him to sleep earlier, and I can't keep him awake; this kid will fall asleep sitting up while eating dinner! How this will work once he has preschool two days a week is a mystery!
Jessica, wife of Marc and Momma to Nikolai (10) and Nathaniel (9) and Olivia (3).