In our family we have a tendency to repair rather than replace our things. Which leads others to offer us their old and or broken things. Since we don’t like to waste and have repair skills, we usually accept their old and broken things. (which is part of why I have too little space in my house and sheds, yes, more than one shed.)
A friend who has 3 sons all within a year of each other (twins and a singleton) passed down their 3 most recently outgrown bikes. Being the former bikes of 9-10 yr old boys you can imagine they’ve seen their days of wear and tear.
We have this thing we do called frankensteining where we take the good/working parts from like items and put them all together to make one item that’s as best as it can be with the parts available. The bikes were no exception. First find the best frame with the best brakes attached. Then swap out parts so that it has the best seat, tires, chain, and handle grips from the three bikes.
And after you’ve fixed what the kids did when they were “helping” you do the repairs, it’s ready to ride! A freshly rehabbed bike!