I'm confused - "since her parents work" she's at your house all the time? If you weren't taking care of her, who would? Is there a sitter letting her come over all the time while she gets paid for doing nothing?

I agree with the advice of taking her only for free things, or to mention to her parents "We'd like her to come to the zoo with us if that's ok. Her admission is $10 and since we'll be there over lunch she'll either need a packed lunch or some money to buy." I'd also send her home at meal times and say she's welcome to join us again when we're done.

A guest is a guest, like Lil said. If you don't want to pay for her, don't take her with.

There isn't a really polite way to say "Hey, your kid's a freeloader, pay up!" Since you've been footing the bill thus far, it will be a little awkward to ask them to start paying now, no matter how you try to phrase it. Depending on what they are like (and given that they haven't even tried to contribute thus far, I'd say they aren't the most thoughtful or clued in people), they may get huffy and your son may lose a frequent playmate. Something to think about.

I think it's OK if you have something bigger planned like a trip to the Zoo, to let them know in advance how much it will cost. But for other spur of the moment things, you should pay and not expect to be paid back.

Yeah it sounds like your watching these kids anyway and not being paid? I don't understand the part about, "Since both of her parents work, the children usually hang out at our house".

Anyways, if you can't afford to pay for her when going out then stop inviting her! If the parents don't have enough sense to give their child money, maybe they are broke too, your not helping by inviting her out. If money is tight just invite her to free things or only to the house to hang out.