What size bicycle should a 9 year old buy?

24" is a wheel size, not really a bike size. Not all 24" bikes are the same size, and not all 9-year olds are the same size.

Basic rule of bike fitting is that there should be 2" or more of clearance between the crotch of the rider and the top tube when straddling the bike. Then the saddle should be at such a height that the rider's leg is almost fully extended with the foot on the pedal in it's lowest position.

Any bike that allows for this has a chance of being a decent fit.