do they have a regular sleep schedule? routines are really important, especially for adhd kids...
there's a great sleep book i used for my son - obviously your kids aren't infants, but the techniques might be helpful. it's called healthy sleep habits, happy child by dr. Marc Weissbluth. the gist of it is getting them on a regular nap and bedtime schedule - making sure the room is dark, no interruptions, etc. it's not easy - the kids will resist, and your life will revolve around their nap/sleep schedule until it's established, but if you're really at the breaking point it's what needs to be done. i know before my kid started sleeping through the night i would have traded anything for a good night's sleep!! i would guess at age 2-3 they still need at least one good daytime nap, but a consistent (early!!) bedtime will be more important. what time do you put them down for bed? if they cry, etc. what do you do?