I agree with playfelt about the dangerous UV rays in the afternoon. I take my dckids outside all morning but we're back inside by 11:30am. Due to varied pickup times in the afternoon I let the children freeplay in the toyroom after naptime and snacktime. This also gives me time to clean up my home a bit at the end of the day so I don't have to do it all after work which helps me not to be so exhausted and burned out by the weekend.

I wouldn't recommend trying to change naptime drastically as you mentioned because that could really set your children's clocks off kilter badly. I've changed naptime around before due to the ages of enrollment but I nudge the time 10 minutes one way or the other every few days so that they get used to the change slowly.