I did it all the time when I was in early elementary. And I loved it! For me it was just another form of playdate. We just "played school". I wouldn't have any issues with my kids tutoring other kids if THEY wanted to do it. Not just because I would tell them to do it, of course. I'd think the child who needs help might find it easier to take help from another child that an adult.

I am surprised by all the "get the child a paid tutor" responses. Nothing wrong with helping someone just for the good of helping someone.