Has anyone used one?  Bought one for a child’s school back back?  I was thinking about the ar5000 phoenix with the 3+ plate (heavy).  Are there other options?

In 3A soft plates there  are a lot of options... from just buying a panel, to getting the whole set up.

Premier and Vertyx

click here for armor and backpacks

Soft armor, yes.  Heavy plate, in a backpack for a kid, no.  It'll be too heavy for the kid, especially with books added to the pack.

https://www.ar500armor.com/phoenix.html

At ~5lbs, a bit heavy but tolerable for a teen in reasonable shape.  If your child really needs body armor at school, I would move or at least change schools.

I have soft armor in a vertx bag for work (school). I like it, fits well, adds like a pound so not crazy. I've flown with it without any drama including a solid med kit. It's not as comforting as a PC but it's next to me all the time and out of the way.

Sort of. I was gifted some old IIIA soft armor inserts, and a few years later, some very first generation AR500 armor plates, something like Level III. My son's school had just gone to lap top only, (all books were loaded in the computer, kid issued same completer every year of high school), so the backpack was MUCH lighter. It had a laptop pouch up front that didn't fit the school one, so it was a simple matter of stuffing the metal plate in with the soft armor wrapped over it, kinda like a spall shield. The weight didn't bother him a bit, and after he graduated I repurposed the armor to other uses.