+1 on simply rebuilding the existing BB using loose balls and the original one piece cranks. There's no need for anything more expensive.
If you want to upgrade there a number of conversions for BMX bikes and you can get as fancy as you like.
I'm amazed that the shop coiuldn't be more helpful, bottom brackets like yours are about as common as sliced white bread, & if they sell any kids bikes at all they should have been able to either sell you the correct retainers, or at least sell you the balls with the advice to pack them loose. Hopefully you were dealing with a kid there, not any sort of senior staffer.
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