Personally I would not wake all the children. Make your mandatory sleep time from 12-2 but allow all children to sleep until 3pm unless they wake on their own.
I think it takes a lot of time and practice for children to learn to be quiet. At 2.5yrs I wouldn't expect most children to be capable of this.
Personally in this case, with a family that has already asked so many favors I wouldn't be caving. Be firm...tell them your old nap policy stands, they can walk or accept it. If they are getting more care for less they can deal with a child up until 9pm.