Family Sharing

I'm trying to set up my child's new iPhone SE. We kept the same number from their old 6s but the verizon store suggested we create a new Apple Id because of parent lock issues on the previous one. I'm trying to delete my childs old apple ID from my family share but it says I can't. I need to delete it to add the new one because with him I already have 5 people on it and that's the max.