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If you've been called to court, you'll have to go. However, you should have all evidence that you've given the child all of her possessions and that the mother is doing this simply to harass you. You can ask the court to award you costs (travel expenses, lost work pay, etc) of having to come and defend this frivolous action.
You certainly want to have the document she signed. Also, have the girl testify that you've turned over all of her things.
Unfortunately, I think you need to go to court. However, you should ask for costs and expenses associated with the hearing as she is doing this to cause you trouble without just cause.