DS(7) usually does 5 minutes every morning, straight after brushing his teeth. He came up with this system himself, so I don't usually need to remind or nag him (astonishing! I never did regular practice for any of my instruments). But he does occasionally skip it at the weekend because he's less in a routine on non-school days. I can imagine he might quite often want to skip it in the holidays, which I think is probably fine, although I worry a bit about him loosing his hard-earned callouses if he doesn't touch his guitar for 6 weeks.
What do your kids do about practice over the summer break and what do you do (if anything) to influence that?