My 3 yo wore her disney princess pajama dresses while we were there. I saw other girls doing the same.

Here's an example of one -

https://www.target.com/p/girls-39-frozen-elsa-nightgown-blue-s/-/A-85288263?ref=tgt_adv_XS000000&AFID=google_pla_df&fndsrc=tgtao&DFA=71700000012510715&CPNG=PLA_Kids%2BShopping%7CKids_Ecomm_AA&adgroup=SC_Kids&LID=700000001170770pgs&LNM=PRODUCT_GROUP&network=g&device=c&location=9061284&targetid=aud-1739091817601:pla-1460123162373&ds_rl=1246978&ds_rl=1246984&ds_rl=1241788&gclid=Cj0KCQiA_bieBhDSARIsADU4zLc3gC7Wp7CIJR29KARQLMTF5L8iJuqeiX4ah6InFocMUIQycnGax6MaAmYgEALw_wcB&gclsrc=aw.ds

OP I am from Florida and have been to Disney way too many times. While I do not live there anymore I still go with my sister as my niece is still young.

My DH worked on the original website for Disney and their cruise line, so went spent a ridiculous amount of time there.

Tens of thousands of kids go to Dinsey parks daily, my guess is about 1-2% wear princess costumes.

Your child should wear what they want it is a very long day.

Some kids there do this but I definitely wouldn't say it's common or everyone. My daughter would have never wanted to and we brought her around that age. She is really active and doesn't like wearing dresses. She also never was into princesses.

Who cares what your MIL thinks. Talk to your daughter about it. Mine would have had zero interest so it would have been an easy conversation. But if yours likes princesses, then think through options with her.