OK, just to clarify. If you are using IR and releasing the ball properly. Isnt that considered a drop for the fastball??
I understand what you are saying and I use to have the same question. I consider a FB that is lower in the zone, just that...a low FB. Once your DD really can throw a drop you will see the difference. I can catch low FB all day long. When DD throws her drops effectively I get anxious and typically don't sit with my legs wide open facing directly towards her and I only like to use a Softie balls to catcher her drops.
My opinion is to go with drop ball next....fairly easy to learn and very effective pitch. A drop curve is a good pitch to have and just as easy to learn as the drop IMO. DD and I use the terminology...tight wrist for drop and loose wrist for drop curve (credit to sluggers for those terms). I would work on drop, then locating the drop, and then drop curve. She will throw a few drop curves by accident when learning the drop. If you need help with mechanics of this pitch Coach Pauly has a good video on his site that can help. My DD is 12 also and the drop is her go to pitch.
Boomers, my memory isn't the best...but I think I remember you posting videos of your DD a while back...maybe something with her arm circle, etc. Anyway, I think she looks awesome...great work!