well you could do what my mom did to my brother... he wouldn't go to bed til 2/3am every night and sleep til 1/2pm during the summer so she would wake him up everyday at 7am (8 at the latest) and tell him 'we have things to do its not my fault you stayed up all night.' and then we had the rule of 'either play outside or help me clean' so we stayed outside playing most of the day. after a week or so he was to tired to stay up and was in bed at a decent time.
I agree, good answer. They need to stay on a routine, even through summer. Send them outside to play all day (or swim) that way they're tired when they come in. My son has a terrible time falling asleep (ADHD Meds), so his doctor recommended melatonin. He takes 3mg every night. Benadryl works nicely too.