First I would not get a Nato. Just a lot of extra strap that the kid has to mess with. I keep my Submariner on a cheap military grade strap that goes under both spring pins just like a Nato but also has a traditional buckle. Here is a link and thta website has other straps. However I don't know if they are still in business but you can see other straps
https://westcoastime.com/military-field.html
Another link with lots of straps - I have the Maratec Mil strap for my Sub. No idea of how good/bad the website is - found it with a search
https://countycomm.com/collections/watch-bands/products/maratac-mil-series-bands
As far as spring pins - have other Maratecs on a Seiko dive watch and stuff like that. No issues. The beauty of the under Nato or the mil strap is you can loose a spring pin and not the watch. Rolex claims their pins are better but who knows. Your main issue may be removing the old band or strap. I have a "classic" Sub so I have holes on the lugs for pushing the pins out. A lot of newer watches have "no holes" so you need a tool to get the pins out to take the old strap off. Looking at a picture of the Gizmo - no holes but I kinda think it may look good with a Maratec. The one issue that does occur to me is that the kid's wrist is small so you may have to trim any type of strap to get the holes to fit