1. What do you mean "write somethign"?
Does the other family have a lawyer? Do not just "write something". Tell the other family your child will particilate in formal investigation or lawsuit as witness. What is the other family doing? Witch hunting for anyone that might be responsible? It is better to have an investigator ask your child and answer truthfully. Otherwise it is a "witch hunt".
2. If you have to be a witness,
tell the Truth that your child have seen by his own eyes, heard by his own ears.
Do not speculate on what might have happened.
If your child cannot remember, say he/she cannot remember.