I had this EXACT scenario occur a couple of months ago. This was one of my part-timers, who was a little under 18 months at the time.
Sweet little girl, I would take 5 more of her if I could find them!! But when she was tired, she was MISERABLE. Dad would drop her off in the morning and ask that I not put her down, or if I did, only for half an hour!!! Are you kidding?? I discovered one morning when I called at 9:45am (to see where they were, as we had been expecting them for some time), that she was still sleeping! Uh, there's your problem. Let her sleep til 10 o'clock, of course she isn't going down until 11 at night.
The first day, I did wake her up a little early (although she had longer than 30 min) and it was awful. After that, I would let her sleep as long as she needed. I freely admit that I fibbed, and said "Oh yeah, I woke her up." Her time here was very infrequent, and I doubt I did any damage in the long run!
Had she been a full timer, or a regular part-timer, I would probably have discussed it with the parents. I wouldn't have felt right about being dishonest on an on-going basis. I would have discussed my personal opinion on the matter, as well as our "everyone has quiet time" policy. Children under 3 WILL stay in the playpen/crib/cot from 1-3.
Chances are, she is having some "reaction" to the new childcare situation. Mom and dad are tired, and don't want to deal with it. The easiest person to take this out on? YOU, unfortunately.