MyFamilyArchive allows the addition of a child to a couple or to a single person or parent. 

First, as with any Tree Edit, put the tree in Edit Mode. To do this, click the three dots in the top left corner of the tree screen.

Then click on a person on the tree.

If adding a child to a person with one or more spouses, the app will ask if this is the child of this one person or of one of the couples.

If you are adding a child to a single person, the app will simply add that child to the single parent.