Young adult book from 80s, pretty sure it's not "The Great Gilly Hopkins"
I remember reading this book in the 80s. Maybe early 90s. I don't remember if the FMC is in foster care but she has a flaky, "free-spirited" mom who leaves her a lot? I want to say the girl goes to live with a paternal grandma. She has a picture of her bio dad and the only thing I remember of that is bright red hair. Parents were maybe hippies? There is a scene where the girl is remembering her and her mom at a McDonald's playing a game where they try to guess people's orders and with a fat person they joke "big mac with a large fry" "and a diet coke".
Bizarre memories but thats all I've got. Been bugging me for weeks.