Help me ID this teen novel?

I’m not sure why this is driving me as insane as it is, because it wasn’t even a terribly good book, but somehow about a week ago I had a sudden vivid recollection of it, and since then have been trying everything I can think of to remember what it was called, but my brain just started leaking out of my ears onto my desk in shiny little puddles, and when the pixies showed up to go swimming in it I realized it was time to ask for help.

Here’s what I can remember. I read it probably in the mid-late 80s, possibly early 90s. I imagine it being sort of middle-America, definitely not urban, and it was about a teenage girl who lived in a foster home where the foster parents were foster parents only for the cheques. They treated her a bit Cinderella-y, and I remember the father drinking beer in a lawn chair. There was also another foster child there, a boy maybe around 2 or so who was of mixed race, and I think the parents would make racial comments to him, and the teenage girl was the one who mostly cared for him.

One summer day they order her to take the little boy and go to the store for more beer, and she takes the money and the baby and takes off, I think on a bus but maybe hitchhiking. I can remember her going to a store to buy food and meeting some other runaways who take her and the baby to an old abandoned building they live in, and they spend the night on an old dirty mattress while the baby charms everyone. Can’t remember why they left there, but they did, and found what she thought was an abandoned farmhouse, and she decided she could raise the little boy there. But then someone came and scared the crap out of them and they were locked in a bedroom all scared, when the police arrive and save the day, and everyone lived happily ever after.

The cover of the book had a picture of the girl carrying a backback and I think looking over her shoulder, with the little boy clinging to her leg.

I’ve tried everything I can think of – does anyone have any idea what this book is?

I don’t know either, but you could ask these people. Or these people.

Thanks a lot!
I’ll do that!