Ok, so you can add children, but you have to do it in the widget. Got to the widget graph and create a function called something like addTimeWidget. Create the widget there and you can addChild, get child and more with some of the widget components.
So if you want this widget to be added, you can either call the function from the onCreate in the UIMenuBackgroundLayerWidget or you can drag off where you were trying to add child and call the function AddTimeWidget.
I said you “had” to do this in the widget, but you probably can find a way to do it outside the widget. In my humble opinion, it is best to add a child widget in the widget blueprint.