Initial thoughts...
Based on age and not knowing if radio will be a continued interest, I would be inclined to stay in a lower price range. If a radio is accidentally left out in the rain, dropped and broken, etc., not too much of a worry. Accidentally left somewhere, not too much of a worry. If it's something that's grown out of as they grow and their interests change, not too much of a worry. Are they going to have regular use or only when they're at the grandparents place? Alternatively, as they use them and develop an interest, show proper care, etc., maybe upgrade to something of a higher quality with more options.
I would stick with something preprogrammed, easy to use (no indepth menu to navigate, no button combinations to memorize, etc.), easily recharged (USB-C), and flexible antenna (not always best but stand a better chance if activities" become rough). Software programming is popular so a programming cable is a must.
I'm usually not much of a brand recomender nor do I know anyone's budget. If you're looking to stay lower price for now, Amazon has a lot to offer that will work. If you're willing to go higher in price, there are several brands worth mentioning... Retevis, Midland, Wouxun, Radioddity are a few.
Just a few quick thoughts...
Whatever direction you go, hopefully you'll bring some younger folks into radio and I look forward to hearing from them on the repeater.
Andy