In a book about Japanese manners for children (written in Japanese) there is a section which has basic "言葉のマナー", and two versions of several phrases: normal and more polite.
It says that in response to ”ありがとう" the normal (less polite) thing to say is "うん", with the more polite ”どういたしまして”.
This translates to ”うん” means "You're welcome", which I've never heard or seen written anywhere before. I guess the indication is this is said in an undertone as sort of a 相槌?
I thought いえいえ was another common way to say "You're welcome" but I guess children don't use this.