Prevent from Adding & Removing record when Parent table condition Met

You also might be able to do something either at the FORM level or post save pipeline level by using the parent-child relationship to store the number of "child items" are presently saved and tied to the parent. (#-Children). 

Then in the Parent-Child relationship besure to include both the Parent Status and #-Children 

Remember that a detail record doesn't count as saved until it is saved, so your detail form PREVENTS save before saving with the parent status = approved and the #-Children = x.