If they mated states that it applies to "non-canon children of two known characters". What do you do if there's only one known parent? For examples, just look at mother_and_daughter chartags:1 and most of the fall into this category.
I've used if_they_mated if the non-present parent was mentioned or easily identifiable by the child's features. However, if the child's just a miniature version of the single parent, then tagging it if_they_mated wouldn't help anyone.