I think just telling them. THey may not understand all the details, but they will know enough to begin to protect themselves and others.
"These are called private parts, no one but you and Mommy see them".
Your body (your hands, feet, tummy, is yours)
When you hear anyone say NO, it means NO.
If you are worried, or not sure, talk to Mommy or a teacher or other adult.
Speak up means to tell someone if you are not sure or if you saw something bad.