Oh boy, this is so typical of that age group. Well, I have two sons, 21 & 11, and a daughter 8. (all from the same marriage). I'm just giving you my stats so you can see I'm really a mom like you
Here's what we found w/ our kids:

1.) Limit chocolate, sugar, absolutely-no-pop or juice before or anywhere near bedtime. Four hours before seemed ok for a snack. Milk has lactose, which is sugar.
2.) About an hour and a half before bedtime, have a 'bedtime slowdown routine' and stick to it. Kids seem to need a 'transistion time' to get into sleepy mode. It gets tricky, because some kids actually find their bath exhilirating, not relaxing. You have to observe your kids; this might work out because you can read to one while the other one is having a bath. If you don't have a partner, you may have to adapt this.
3.) Here's another thing-be watchful of the reading materials you choose for before bedtime stories. Some children's brains get excited by an exciting story, or words that produce questionable or scary imagarey.
4.) One of my kids liked to have their feet rubbed for a little awhile.
5.) Associative routine really will work if you stick with it.
6.) Make sure your daycare provider is not keeping them 'hepped up on junk' all day long, or not allowing quiet time for a toddler's nap. I think a young child's central nervous system can actually be damaged by over stimulation and poor nutrition over a duration of time. Maybe I'm wrong-but after 15yrs. it sure looks like poor results are poor results.
Don't lock the door in the dark (I know you're just frustrated)-wouldn't that make you terrified, that the person who's suppose to be teaching you and keeping you safe suddenly turned into a 'dangerous monster'?
Also, you might want to look up all the literature that has 'Child/Children & Challenge' in the title. Good luck-remember you love that little one