Adult starting rumors about a child

What would you do in this situation? An adult (ex family member) has been trying to start rumors about my young school aged child.... by making up stories and telling another family member, saying all the school parents are talking about my child, knowing it will come straight back to me.

I am worried if 'said rumors' start to spread at school which could potentially lead to bullying and alienating my child.

Note: rumors are completely untrue, not even close to an ounce of truth in them. It makes me laugh but I am more so gobsmacked that an adult would stoop to this level and the potential impact it could have on my child should it escalate 🤦🏼‍♀️