K.H.
Hi
I would just say to her that you & your son love having her kid over to play because they get along so well , but during the school holidays you don't work as a daycare as you like to spend time with your kid doing family stuff. Say you are happy to watch her boy sometimes if she really needs you to but most days you already have plans and you cannot look after her son while she pops to shops or goes to work etc.
If she takes offense then that really is not your problem , you shouldn't have to feel like you are walking on egg shells with a neighbor and she should know better than to just bring her son to you and then go out somewhere , if she needs daycare that much then maybe you could give her a copy of your rates!
Good Luck
K.