Hi E.,
I'm not sure what time your kids get up in the morning, but I think bedtime should be about 7:30 or 8:00 if they are up at around 7 or 8 a.m. and that's with a nap planned in.
Try writing out a set schedule for your typical days. Example: Breakfast, play, lunch, naptime or quiet activity, outside play, dinner, bath, snack, bed, etc.
Even if you keep her up all day, I'm betting that by 8 at night, she has already, much earlier, "hit the wall" and is overly tired. No 2 yr old can be on 'go' for 12 hours straight!
Personally, I would establish an afternoon naptime earlier than 4, say, around 1:30 or 2 in the afternoon and make the rule that they both must stay in bed/crib for at least O. hour. They both might just crash. If they don't sleep, at least they have had some down time and bedtime should be at least an hour earlier that night if they do not sleep at naptime. If they do take a nap, STILL enforce that bedtime is at 7:30 or 8:00. Even if they don't SEEM tired/crabby/etc. they ARE.
It will make it easier if you can write a schedule, show them, and stick to it every day.
If the 2 year old conks out at 4, don't let her sleep more than O. hour. Still keep bedtime at the set time. I'm guessing that with all that's going on right now, you're probably glad to see her fall asleep whenever the time so you can get at least a little peace and quiet and get some things done. But don't let it happen. Plan your stuff around their set naptime/quiet time in the afternoon.
It is a HUGE myth that very tired kids will sleep well. They don't! She will sleep better at night if you put her to bed when she does not seem sleepy.
You may have a few rough nights getting her used to the idea that 8:00 means BED but it will pay off in the long run. A kid is NOT going to tell you they are tired and need a nap. They just zonk when incredibly over-tired--like your daughter is doing. Better to get her in a routine so that she doesn't ever get to that point.
I don't know how you are functioning! I have O. son, soon to be 6, and if he is not in bed by 9:00 p.m. at the latest I am throw completely off MY schedule! I need that time in the evenings to myself and to get things accomplished. It must be NUTS to have a 2 and 3 year old running around the house at midnight.