This situation is a perfect example of the body and muscle memory.
My 9 year old player has the same problem. I can get her to "dry throw" just fine until I put a ball into her hand and she immediately drops her elbow.
After a few reviews and looking up different ways of trying to correct her...
For the money, I think Mizuno makes a pretty good mitt or glove.
The mitts and gloves with the Power Lock closure break in pretty easy.
I think the GXS100 is pretty good for younger players and at 39.95 not a bad price. and the Velco wrist strap makes it nice.
The GXS91 Model would probably be...