You need to enter your widget blueprint and make the component you want to be the parent set to “is variable” like in the picture above.
Then you have to go into the event graph of your widget and (if you want) create a custom event to trigger the add child (The reason I say custom event is because it can be executed from other blueprints) and to add a child to the parent you use the node “add child” like in the picture above. when you have that node you need to make another widget to have it create in your parent. (NOTE: your component must be able to accept more than one child or else it wont create the new child)