What is the best approach for my child to prevent mono?

Difficult. Mononucleosis is a viral infection, caused by epstein bar virus ( ebv) or cytomeglovirus, mode of spread is like cold , once infected stays in the body forever , how you are going to prevent kid from kissing ( also called kissing disease, ask for a monospot test ?. It is part of growing up.
Avoidance. Try to avoid those known to be sick and always employ good hygiene such as handwashing.