Ravi, the youth model should work great for her since she's smaller. As for upgrades it depends on the model. Several manufacturers make muddy girl shotguns like the Savage/Stevens 320 youth, Mossberg 500 youth and couple more but most likely it'll be easy to purchase and swap out the stock and barrel for a full sized version. If you post again with the model number or upc we can point you in the right direction. Hope this helps.

You would be able to upgrade to a full size version with a Steven's Muddy Girl but personally I'd go with the Mossberg version as you'll have better options with regards to aftermarket support.