Hi there!
We have noticed the same thing when looking for cute outfits for my six-year-old son! What we have done the past few trips is worked vendors on E t s y (can we even mention that site? sorry if no...).
Anywho, I find a nice pillowcase dress for my girls (8, 3) with a nice Disney fabric. I then Private Message the seller and see if they are able to make a bowling shirt in the same fabric. Some sellers are willing to do that for an additional cost.
I'm not crafty at all, but my mother-in-law can sew and she says that the bowling shirts aren't too hard to make.
Just keep in mind, it may cost a bit more if you want a bowling shirt because I think it's more time intensive to do those rather than the pillowcase dresses for the girls.
I think there's also a crafty board that has a thread on sewing...some of the folks on there may also be able to assist if you go this route.
If you need a recommendation of a vendor we've used in the past, just send me a PM. I hope I can say that on here...if not, sorry again!!! I know the rules are strict about certain things, but I'm hoping I'm not getting into any hot water with this post.
Good luck!! And, have an amazing trip!
BTW, my nephew is 8.5 and he loves the bowling shirt we designed for our upcoming family cruise. The fabric is super cute Mickey Mouse!