Hello Maribel! Welcome back to the Disney Parks Moms Panel.
Congratulations on your happy surprise! No doubt the Endless Magic of Walt Disney World Resort will amplify the already fantastic news in store for your sister-in-law.
More than any other entertainment venue I have visited, Walt Disney World Resort is very permissive when it comes to bringing in outside food, drink, gifts, etc. At every Walt Disney World Resort Theme Park and Water Park, you will have to go through
Security Clearance and the Main Ticket Gates. During Security Clearance, any bags, gifts or balloons you are bringing into
Magic Kingdom must be inspected and cleared with a Security Officer before entry will be granted.
As per the
Park Rules and Regulations, the surprise you mentioned in your inquiry should be fine to bring into Magic Kingdom. However, it is up to the discretion of the Security Officer that checks your surprise whether or not what you are carrying contains any "items that may be harmful or disruptive" to other Guests.
Only
Disney's Animal Kingdom Theme Park specifically prohibits balloons from being brought into the Theme Park due to the danger they could cause for the wildlife.
If you are looking for some assistance in planning your special surprise, you may want to look at the offerings from those who
specialize in Endlessly Magical surprises at Disney's Floral and Gifts.
I wish you the best of luck with your surprise and fantastic news!
~Todd