No usually I don't. But I can imagine plenty of people do. I'm less likely to assume it's theirs than I would with an adult (where I'd usually assume the woman pushing was the mum).
EDIT: Looking at other posts, I do agree that the clothing etc could make a difference, although it shouldn't. It might also change depending on what they're doing- a weekly shop might make me more inclined to think the girl was the mother, as opposed to a stroll in the park.
Although I don't mind- I don't think having kids young is a good idea, but it's up to the girl and I don't know the situation. Maybe if I was more judgemental I'd be quicker to assume?