Hi Carlie! Sounds like you've got some 'spooktacular' plans for this October. I am so excited that you are planning on attending Mickey's Not-So-Scary Halloween Party. Halloween is one of my favorite holidays, and at Walt Disney World, it is extra festive with all of the fall decorations and pumpkins.
Details still haven't 'materialized' for Mickey's Not-So-Scary Halloween Party, so I would suggest keeping a Mike Wazoski-eye out for any updates on the Disney Parks Blog. I know that last year Stitch did not make an appearance at Mickey's Not-So-Scary Halloween Party. Stitch also used to have a dance party in Tommorowland but it is no longer running. To guarantee a meeting with Stitch I suggest making breakfast reservations at
'Ohana. Lilo and Stitch will come around for pictures and an autograph while you enjoy a delicious breakfast. This is your best bet in securing a meeting with your daughter's beloved Stitch.
As for a Stitch costume, I don't remember seeing any in the parks, but would recommend looking around at the Disney Store online. If you still don't have any luck, I would look on Amazon. I found some Stitch costumes while doing a quick search just now, and you could order before your trip so she could wear it at the Halloween Party.
If you need anything else, please feel free to float on in. Have a 'frightfully' good time at Walt Disney World!