It's hard to believe they have had magic bands for what, 6 or 7 years now, and they haven't figured out how to utilize the magic band technology in any useful way, but this is the most obvious. They could have a "height measurement station" by guest services in each park, take a picture and a height measurement there (which families could do PPO, and ideally just once at the beginning of the trip), and then call that up at a scanner at each ride that has a height requirement. If they want to be extra safe, they could require a finger scan right before you board, to ensure that the band wasn't switched.
None of this would be free for Disney, but it would be a unique, memorable solution, and it would improve the guest experience SO much -- it would remove all uncertainty for parents, it would actually make height measurement/confirmation fun for the kids, and it would reduce the lines at FP entry points, which are often stopped up with measuring kids.
I can't believe there is not a unit in Parks that spends their time thinking, "We spent over $1bn on magic bands, how can we use the bands to improve guest experience, or in other ways that will set Disney apart from every other theme park?"
Our boys are 9, 6 and 2 right now, so we are still in the thick of this. For families with a few kids spread out, there is maybe a 10-year period where they are watching at least one of their kids get measured every time they go to a theme park.