Kidfic is a sub-genre of fanfic that features children in some way. This may be as children of canon characters (whether they are a part of canon or original characters), sometimes known as next generation fic, the characters caring for someone else's child eg babysitting, the characters themselves as children, or the characters having been de-aged. Stories from sources where the principal characters are children, such as Harry Potter, are not usually regarded as kidfic unless they involve a trope such as de-aging.
Fancake has a round of people reccing kidfic stories from various fandoms.
Some Examples of Kidfic Zines
Some Examples in Fanfics
Some Kidfic Fanart