Apple Watch Share my line for Family member

Situation: I have an iPhone and an Apple Watch. I have a 3year old that I would like to be able to find if he ever got lost and I was told I could add an Apple Watch SE to my plan for 10$ a month.

I went to do that, and they said my son would need a separate line at 20$ a month or else he would get all my texts and calls.

can I disable the inbound calls to his watch and still have him on my family plan, or would he actually need his own line, which makes this a little more expensive than I wanted to go.