Depending how often you fly or travel you can get a trunki snoozihedz which is a blanket and inflatable pillow and the blanket connects to the pillow. It all folds compactly. Sometimes they do have blankets/pillows on domestic flights but you need to ask.
Has she been on a plane before? I would play 'planes' with her and your DS just so they have a bit of an idea. Try the library for books on flying too. If she understands she can't get up unless to go to the toilet it might help. You can take playdoh but as a PP said it would be messy. I would take a small tub just in case - be prepared for them to inspect it though.