Children also get exposed to much simpler language and get to learn a language more bottom up.
Also somehow my impression is that pronunciation/accent really is harder to adopt as an adult than as a child.
Generally, this works well for kids and adults without being demeaning but it's also important that everyone understands their role in the situation beforehand if possible.
Regarding accent, I think it really is a matter of practice and will. I agree that it's harder as an adult, but we shouldn't allow people to use that as an excuse if they want their accent to improve. By no means is that a requirement, but it is something you should ask your adult friends if they are learning your native language. Though, I would argue this is probably not even worth it unless they are at a B1-B2 level. If they come from a similar language family (germanic, latin, etc) as you, then perhaps working on their accent at the beginning will prove fruitful, but otherwise, it's probably a lot of new sounds that they need to learn how to even say "correctly" before they can start adjusting how their mouth moves. For me, I've found that mouth training a little easier when I'm not spending 10 seconds formulating a reply in my head.