Z. - You don't mention what your approach was for moving their schedules back. While I have never had to deal with a 2 hour difference but when my daughter was off-track by 30 mins to an hour, I basically moved her bedtime 15 mins at a time over a week or so.
In your case it may take longer, here's how I would do it:
Shift their entire schedule by 15 mins starting with bedtime and throughout the day (meals, naps, etc.). Stick to that 15 min shift for a few days until it seems to have stuck. The follow the same process shifting the schedule by 15 mins each time until you are at the schedule that seems to work.
Also you mentioned that their bedrooms are darkened. Most people's level of sleepiness is sensitive to the amount of light surrounding them. Maybe you should make sure that their rooms are light enough in the evening until bedtime so that some light may help to keep them up. My daughter is sensitive to the light so in the summer she stayed up a little later than usual and now she is already on her way back to falling asleep a little earlier.
Good luck!