Welcome back Stacy-Felice! Thanks for stopping by with your question.
For safety reasons, Disney has created new guidelines for wearing costumes at the parks. Guests age 14 years old or older are not allowed to wear costumes at Disney Theme Parks, except for special events at Disney Parks. You can find a list of the
costume guidelines at Disney official website. Also, it has been posted recently at the
Disney Parks Blog, a video about Costume Guidelines for events at Disney Parks.
I believe you will not be able to wear your Minnie Mouse onesie at the parks if it is a regular park day. I suggest you
call Disney World and ask about this particular outfit.
Please, come back to visit us again if you have further questions.
Have a wonderful day!
Gina V.