Kids and phone plans

I want to losen the rains on my kids, which is hard for me, as my sons father has a history of snatching our son. Long story, he's on a no contact order. My son is 11and my daughter is 9

We have a little park about a 5 min walk away, and a bigger one about 10mins away. Today, for the first time, I let them walk to the park alone for a play. The whole time I was in panic mode. I thought about getting them a basic prepaid phone for when they go for walks, so we can contact each other if needed and track them on it.

What basic phone and plans would you suggest. It would be a shared phone for sleep overs, or when they go play at neighbours homes and such, it doesn't have to be anything fancy. When master 11 starts highschool he will have his first personal phone