How to restrict taxonomy term reference with certain terms?
You seem to be asking 2 questions here: 


Restricting Terms: You can edit the entity reference field's Reference method to use a new custom view that filters only the criteria you're looking for. 
Parent/Child Widget: The Hierarchical Select did have this widget functionality in D7, in D8 there are a few modules out there, like Simple hierarchical select &amp; Client-side hierarchical select, that have similar dual-selection functionality.