If I were you ....

1. Send a letter to everybody to thank everyone includes A and his family for the opportunity to let your kid in.

2. Suggest another special test between your kid and A to determine who gets in the team if A wants.

3. If no answer after 2, just let your kid join the team and no need to worry about it anymore.

4. If A's mom still keeps bitch about that after 1 and 2, just give her a finger....