How little is little? I have had kids in care up to around 18 months that still need a morning nap (albeit a shorter one). I do find that most of them give it up (or can be adjusted) a little after the first birthday.
If it doesn't interfere with your day-to-day, and mom and dad are really insistent, AND you think that this child would benefit from keeping the morning nap; I would keep trying.
I wouldn't enforce total silence from the other kids, but maybe that's not the time to play loud and crazy games. Maybe it can be coloring or craft time, or some such thing? If little one refuses to sleep at all, at least they have had a bit of a rest/quiet time.
I like that you have asked them to adjust to YOUR schedule. So often parents want us to adapt to theirs, which is (most of the time) not feasible. The problem I can see these parents having, though, is he/she is likely falling asleep on the way home, or so cranky and irritable that they are stressing out!