DR Jennifer,3, 5 & 7-I love those ages.
The situation does have the potential to be awkward, especially since you were caught off guard. You say you would never charge for family, this family may wish to be considered as such now she lives so close to you. She may also feel hesitant to mention money up front. If you expect something I think you should let her know beforehand.
Do you want a semi-regular babysitting job? If so discuss money. If you aren’t sure, go ahead & sit without mentioning money & see what happens. You can decide to accept the money, if offered, or not. I would make it clear at the time what your future intentions are. If you plan to sit when possible, or only if she is in a bind. If she doesn’t offer anything & she asks you to sit again, you can then mention wanting payment if you sit more often.
It really comes down to whether or not you want to sit these children. I suppose you won’t know until you do it once.
Did that help?