Advice for a Summer Nanny?

you are taking them out alot and taking them out cost money. If their parents are fine with paying for all these activities go for it (but make sure they actually like what you have planned). If the parents don't want to pay for you guys going out all the time there is alot of stuff you can do at home. Plus the 12 year old girl is most likely going to want to hang out and go out with friends, and it is very easy to keep a five year old entertained. You could set up scavenger hunt type things in the house, play hide and seak, set up sprinklers, or just give them the option of deciding what they want to do.