Is money earned from helping at a children's party that included drumming and music permissible or forbidden, and how can one dispose of it and atone for the part that was spent if it was forbidden?

The permissibility of money earned from a job is based on the ruling of the job itself. If the job is permissible, the money is permissible. If it is forbidden, the money is forbidden. If it is mixed, it contains a portion of what is permissible according to the permissible work, and a portion of what is forbidden according to the forbidden work.

If the money is forbidden, it must be disposed of by spending it on general Muslim welfare. If it is mixed, the forbidden portion must be estimated and disposed of accordingly.

Since lifting the drum to the stage for children to use involves aiding in what is forbidden, as it is among the prohibited musical instruments, if part of the payment or gift is in exchange for this aid, then that part must be disposed of.

As for the presence of accompanying music in some games, you have no relation to it.