I let my 11-year-old stay up a bit on the weekend and during the summer, but maybe an hour or so late. She wakes up early usually on her own so that isn't an issue. During the summer she would sometimes stay up quite late but really just for special events, not as a rule.
The 4-year-old goes to bed at the same time regardless.
I guess I like to generally keep our schedule because I think it's hard to adjust your inner clock all the time. But I try to have a bit of flexibility.