Family Sharing - same child Apple ID for iPod and iPhone

My son had an iPod and we had the family sharing set up for him, along with setting downtime restrictions, app restrictions, etc. We recently got him an iPhone, using his same Apple ID. I'm trying to add restrictions to his iPhone but it seems like its still only allow the restrictions on his iPod. I was thinking with the same child Apple ID we could use the same restrictions on the iPod and iPhone. Am I missing something? At this point I'd rather have the restrictions on his iPhone instead of the iPod if I can only have one device per child apple id, but how do I do that?