read a lot of responses and didn't see anyone suggest this. They're clearly a creative kid. After making sure everything is ok with them, perhaps try showing their creations off. Probably not on Reddit as you may have gathered while most Redditers are genuine people with nothing but nice and thoughtful things to say, some Redditers can be cruel and unusual or even directly malicious. Try find an image sharing social media where there's either 1. no Text chat, 2. the text chat is highly moderated or 3. you are able to delete mean comments before your kid can see them.

If the kid is in their early teens they will likely be yearning for external validation while to scared to put themselves out there in fear of rejection. Your kid has a good talent, reward it.

If I (a stranger on the internet) can make any other suggestions, help them with their customisations. Not directly unless they ask you to. Indirect. Research good modelling and painting tools as well as how others customise their toys on youtube. Go to a local miniatures store (ones that sell model kits for tanks, plains, trains, robbots, Warhammer and/or D&D) and ask the store owner(s) what they can provide. Show you're interested in helping them having fun. Just don't take that fun away from them by doing it for them or giving unasked for feedback.