Best to charge 'something', which will depend on your personal risk-taking factor. Children oftentimes end up 'costing' more than adults, depending on age and listing. I always do, though the charge varies taking into consideration those two factors.
Of course you should charge them. At least for weekends while they will be there, they will use bath towels, sheets etc...
It is a good idea to have "no visitors day or night" in your house rules so even if they come in the morning and stay unitl bed time you can either charge them or not allow access.