I'm a SAHM and my DS (9) has a friend whose mum frequently asks me to have her son, after school , on a Sunday, while she is at work or entertaining clients. I may add they are a dual income family with more than plenty of cash, it's their decision to maintain two Hugh flying careers without formal child care. Additionally the child is quite lively and fairly hard work, unlike my DS who i find very easy to look after. I am constantly making excuses why we can't have him, but I feel she shouldn't ask.
I do have other friends who I help out more, who are more likely to reciprocate, or whose working situations I have more sympathy for.