@Sean433 No, you aren't missing anything. This is one of those parents who seems to think everything their kids do is perfectly fine. Of course throwing or bouncing a soccer ball inside the house is unacceptable, even if there weren't a downstairs tenant. " That's an outside game, kids" is what a responsible parent would say if their kids started doing that.
I might have sent a slightly different message-I'd have left out the part about possible damages and made the message a little more personable. Like "Hi XX, The downstairs tenant is complaining about noise from a ball being bounced around. I know kids will be kids, and glad they are having a good time, but could you please ensure that ball throwing and bouncing is an outside activity? Thanks for your co-operation."