Yes, anyone is welcome to attend the Expo! In fact, Disney encourages it and the vendors love it, because it means more “bodies through the door.”
Just be prepared to do some walking, as you’ll need to go to the HP Field House for the bib, Visa Athletic Center for the shirt/expo floor, and AdventHealth Arena for any merchandise. It’s a nice tour of the WWoS Complex!
I will caution you that there might not be much merchandise available by the time you arrive on Friday. Between supply chain issues, resellers, and high demand items like Spirit Jerseys, lots of items have been selling out within hours of the Expo opening on the first day. I just don’t want your niece and nephew to be disappointed, but the medal and included shirt are still amazing souvenirs of the run!