There is a plastic electric toy that simulates the sound of a drum when one beats on it with a stick. May kids play with this during the three weeks or during sefirah when music isn’t allowed?
Yes, this is fine – from the sound of the question is does’t sound like music but more like a toy, and if the kids are playing with it and not making music, there is no problem.
If it is being used in a musical way, i.e. for making a beat to dance with, it shouldn’t be used.