I wouldn't think a college student or grandmother would be very INTERESTED in being a mother's helper. In my mind, that's mostly a job for a young teen... if you need an adult to do it, that's more of a nanny or a housekeeper in my mind.
When I was in middle and high school I helped out the same neighborhood families I babysat for. I would help with the afternoon stuff after school or come over if mom was getting ready to host a dinner party, or had to get work done etc. Basically the role is like that of a responsible older child for a family that doesn't have one.
Entertain little kids while mom get's stuff done.
Make, serve and clean up snacks.
Help little kids with homework (if they're in school).
Help kids clean their rooms.
That kind of stuff.
Given the age groups you mentioned, I'm wondering if you don't mean like a light housekeeper? If you aren't looking for a babysitter, you may want to phrase it differently as you search, because most people will think you mean that.
HTH
T.