When I boarded my horse in a University co-op barn that had a lot of new students coming in (and then leaving) each semester, we had a pretty good routine. For each "chore session", there was a sheet that listed each individual job (hay in each stall, water each stall, grain, supplements, hay in pastures, water in pastures, sweep, lights out, etc) with a space to initial. When you did the job, you initialed the spot. Later, if you hadn't in fact done what you said you did, you could be called to task.
I could see a variation on this (which I think other posters - maybe you - have already suggested) working out well for your girls. Since you say that most of the things seem to get done, but not everything in every coop, it could just be a bit of summertime forgetfulness. Posting *exactly* what needs to be done with a space to account for who did it could take care of the issue. You still might need to check up, especially in the beginning, but having everything laid out clearly can make a job with lots of "nit-picky" parts a lot easier. It might be even better if you could get the list laminated and have one out with a dry erase marker by each coop....
I have mixed thoughts about the idea of taking away egg money/making them pay you to do the chores. On the one hand, if the chickens are their responsibility, they should take care of them - being able to "pay you" to do it is a bit of a cop-out if they don't need the money. On the other hand, well, that's the way the real world works, right? If they can still afford to "pay you" to do the chores, and make a profit (somehow!
) then maybe that's "okay". I'm glad my son is only four and still gladly helps take care of our chooks. I'm sure I'll be the one asking for advice in 5 years!