Centerpieces

Maybe it's a regional thing, but centerpieces aren't typically taken by guests in my area unless they're real florals that will die or something like that. I'm not sure there's any polite way to say "please don't steal our centerpieces." I would just have a family member or DOC keep an eye out and let guests know not to take them if they try.

As PP said, most people only take floral centerpieces and in my area, it’s usually after the bride or groom has said to. As for yours, I would maybe make a small card/sign or 2 to put on each table advising that it is a sentimental item or something like that

Hey Brianna! Is your entire wedding Disney themed? That’s so much fun! Do you have any inspo pictures for your centerpieces? I would love to see what you are thinking for them!

As the pp’s mentioned, it more typical for guests to take fresh flower centerpieces so that they do not go to waste or get thrown away. It would usually be after the couples say so that centerpieces are taken. If you are worried about guests taking them home, perhaps you could mention it to some of your family members to keep an eye out for any guests taking one with them. I’m sure they will completely understand.