I wouldn't say it's "very, very low". According to the
CDC data tracker 35.4% of American kids 12-15 have had at least one dose, and 46.9% of teens aged 16-18. It's definitely lower than seniors, but there's plenty of a market out there if they want to pursue it.
I'm starting to think that Disney might sail the Wonder under 95%+ vaccinated protocols (probably still skipping the West Coast season) while allowing more kids on the Dream and Fantasy. It would be an interesting way of serving both those who have kids as well as those who would prefer to sail on highly vaccinated ships.