Hi, Shawna.
I am deeply sorry for your family's loss. Something special to remember your little angel during your family's Walt Disney World Resort visit is a fantastic idea. Unfortunately, Walk Around the World memory bricks in the Magic Kingdom are not currently available and there is not currently a similar program.
Another idea would be to display random acts of kindness while visiting the Theme Parks. Even though this wouldn't be something you can see physically while visiting, the memories will last a lifetime for your family and others!
You can purchase several inexpensive souvenirs and pass them out to other children (with their parent’s permission) and attach a small note to the items such as “You have been pixie dusted in remembrance of our daughter.” You may include her name if you wish. This would work well with trading pins, stuffed animals, balloons, Mickey’s Premium Ice Cream Bars, etc.
Bring small containers of bubbles, like the ones you can find at party stores for weddings, and pass them out to others before a parade or fireworks show. The children around you can blow bubbles while waiting for the show to start. The bubbles will travel upwards to the sky... Also, I bet their parents will be thankful you gave their little ones something to do while waiting!
If you are dining at a Table Service restaurant, you can purchase a dessert and ask your server to choose a family to give it to after you leave. You can include a note such as, “Here is a little something to make your vacation extra special, in remembrance of our ‘sweet’ Angel.”
Wishing you and your family a memorable and magical visit, Shawna!
~Rachel