I'm not sure I would count on there being any wings or kid 'kits' on the flight these days. Used to be they had stuff for kids . . . BUT you can buy some airline wings on eBay and make a little 'kit' yourself. Maybe buy some Business Class amenity kits, again on eBay, and put some kid-centric things inside like some candy, keep the socks, crayons . . . you can make a little certificate of their first flight with name, flight number, route - and have the pilot or a flight attendant sign it? Would make a nice momento, memory. You couild also make your own route map? There are also a few books on Amazon dealing with a child's first flight.

Here is a print your own first flight certificate:
https://www.aopa.org/-/media/Files/A...ertificate.pdf

And here are some books: