My 7 year old daughter has one of those (a Mongoose) and is nuts about going for a ride at every opportunity.

First, I would find a new bike shop.

Second, as has been mentioned, ball retainers are rarely necessary. Their primary function is usually to speed up the production line as they can be plopped in place. If the balls are installed singly the cups must be greases and the balls installed being held in place by the grease as the BB is assembled. This takes a little more time and skill.
If you can't find new parts almost any cheap kids bike has the same BB. Just up an organ donor at the nearest thrift store or yard sale.

My kid love to pedal and I have found that she is actually more like a kicker motor than a boat anchor on the up hills.

On note of caution. When going over humps (think speed bump but bigger) the chainring on the trailer bike tends to bottom out so be careful.