Parent/Children as a tag?

it would make things easier to find it, why is there no tag for it?

You can use parent:any or child:any to narrow your searches down if necessary.

Edit: Unless you mean actual families, there are tags for that too.
father_and_son
father_and_daughter
mother_and_daughter
mother_and_son
parent_and_child

pictures like these dont have any of those tags or is this a feature for subscribed accounts?

http://danbooru.donmai.us/posts/1697124

STAYLOGGEDIN said:

pictures like these dont have any of those tags or is this a feature for subscribed accounts?

http://danbooru.donmai.us/posts/1697124

child:any and parent:any are metatags. They're not listed in the post's tag list, but searching for them will still return the relevant posts you want.

Example for the post you linked: child:any id:1697124