A slightly different take on this.
My kid did a bootleg dive during a boy scout campout. He was on the octo. Totally hooked, even to this day. He is 30 y/o. Now owns all his own stuff.
At ten or eleven just being competent at breathing under water and have a sense of buoyancy is the 'be all and end all'.
All the destination brags are totally lost on me. Who is that for?
I mean, I don't really see anyone bragging - no one suggested Bikini or Truk. Pretty sure an eleven year old is going to have a significantly better time on a pretty, warm water reef than just being underwater in a Colorado pool or a quarry. The OP is exactly right, that some people never dive again if their OW dives are somewhere unpleasant.
Would not go with the Keys unless you absolutely have to stay in CONUS. I like them a lot, and it's great wreck diving, but the shallow reefs are not that terrific (in my opinion), and conditions can be rough with currents.
If you want US territory, I would say St. Croix. I could dive the Frederiksted pier every day for a month and find new things in easy, shallow water. Bonaire probably is the best bet anywhere within a reasonable flight for you if you want a lot shallow, accessible reefs that are good for a new, young diver.