Boy do I sympathize with you. I have the same problem with my 3 year old son, Jessan. I can wake him up at the crack of dawn (to much screaming and whining, which actually seems to happen anyway) keep him up all day, with the plan that bedtime will mean asleep by 9 pm at the latest. But this plan usually backfires on me. He certainly is a night owl which just doesn't work for me.
The part that is hardest for me personally to deal with is that until he started climbing out of his crib and was moved to a toddler bed, from birth almost you understand, he was religiously put to bed at the same time as the other kids, always by 9. He spent the first 18 to 24 months on a very strict sleeping schedule, with no problems. Then, BAMM, problems. I just don't get it. We have a bedtime routine that includes someone reading to both Jessan and his big sister, Sequoyah (4, who has no problem going to sleep or waking up), either myself, hubby or one of the other big kids. He always get a bedtime is coming warning, gets his bath, teeth brushed and then some cuddle time with mom, before the actual bedtime hits.
I wish I had some actual helpful advice for you, but unfortuneately, all I have to offer is to let you know that you're not alone in your dilemma. I'm actually hoping to hear something helpful from one of these posts. So far I've noticed: routine (I've got it, and since you're running a daycare from your home, I assume you have also), limiting sugar and caffine intact (duh), or medicating our children to get them to sleep(I don't know about you, but this rates right up there with putting misbehaving kids on Riddilin or other mind and mood altering drugs-- which comes down to not an option, at least for me). So I guess that leaves us both with deal with it until these kids get older and mature enough to realize this is going to happen whether they like it or not. Not a very pleasant thought, but I guess our only option.
Good luck, I'll be praying for your sanity, please pray for mine.