Rather an expensive sport for that young.

It's an investment in the future of a more accessible America. We already pay for so many sports, is it unreasonable to ask that one of them be handicap accessible? Besides, a lack of consumer interest makes basketball wheelchairs producers more rare which drives up the price. If more companies are incentivized to produce them, then other less expensive versions will be developed. Right now, you only see the prices of the high end wheelchairs. But take bikes for example, high end bikes are similarly expensive, but we also have cheaper bikes that are perfectly usable, and nobody says children should not bike around.

Also what are the injury risks like?

There is no data on pediatrics sports injuries in wheelchair basketball on pubmed, because there aren't enough pediatrics wheelchair basketball programs.