Aren't children supposed to be better at learning new languages than adults, though? There's supposed to be a critical period where language acquisition is much easier.
The fact that vocabulary of a 6 year old child is much smaller and takes less time to remember of course helps
On the other hand, when you are adult , you can actually understand abstract concepts of grammar. It’s just too boring to repeat everything enough times to train your neural network to get it right without thinking.
What children are actually really good at are sounds of language.