Is it a playdate or am I babysitting?

So for the third time this month, an erstwhile friend of Daughter's is coming over for the bulk of the afternoon.

She's a nice enough kid, maybe a little rough around the edges. Daughter was friendly with her during school, but did she play with her at recess? Nope.

Still, throughout the summer, I get these last minute phone calls from her mother, "Can Raggamuffin come over? Like in an hour?" And then I'd have her for several hours. This happened twice during school as well.

Now, I don't mind taking care of other people's kids, but within reason. Heck, just yesterday, I thought I'd take Jules's son off her hands for the afternoon so she could get some work done before she splits for her second honeymoon of sorts. But that's entirely different because:

a) I offered
b) Jules's son is a super close friend to both Daughter and Middle Child and, frankly, he's like my surrogate nephew

But with Raggamuffin, I feel like I'm being used. I know her mom is off doing stuff; the kid tells me as much. I personally loved, "She's going to Toronto with her friend to do some shopping. What's for dinner?"

Are there playdate guidelines that I don't know about? Let's open it up here because, obviously, if there are rules, I'd definitely need to see them published.