At least last time I donated blood in my country (Canada), you could discretely indicate “do not use” by applying a different sticker to the bag. This was done in case someone got peer pressured into donating but didn’t want to reveal something private that would have disqualified them otherwise.
My wife and I had this conversation the other day. Our kid is only two right now, but as we’ve learned, these milestones sneak up on you.
I used my own life as a guide to my opinion, and so landed on age eight or so. That’s around the age I remember being able to go to the park or to a friend’s house within the neighbourhood on my own.
Other questions about how much functionality the phone would have and how much access they would have to it at home are still to be determined.