How to tell someone you don't want their child touching your baby?

What is the nicest way to tell someone you don't want their seven yr old touching your baby? Sunday this kid wouldn't quit asking to hold my baby I was really nice about it, just wouldn't let her. Her parents seem to think I should have let her and pointed out how good she is with babies and acted all offended. 🙄 So if this occurs this coming Sunday what is the politest way to get them to realize I'm not trying to be rude or mean to their child I just don't want my little 6 lb baby being handled by a small child?