The otherside of someone's case, law-suit, whoever it is going to court wants to know what your child knows.
A deposition is usually an "under oath" testimony from a given individual's (your child, in this case) on their knowledge or involvement in what ever the case/suit/insurance investigation.
I'm jut an experienced parent, I really think you need a lawyer.
Because of the age, your child might be allowed to provide a written deposition or privately appear in the Judge's Office / Chamber. But you need legal advice.