I'm really sorry you're scared, but remember - Disney World has to be one of the safest places on earth for children. All the cast members look out for kids (and not just the little ones, either). We're going on Wednesday and my oldest son is 10. Unlike you, he'd kind of like to be separated from us (thinks he's 20!). But we've talked about what to do if he is lost at any point and it sounds like your mom is doing the same for you.
It might help you feel a little more secure if you make up an emergency information card. I did this for all 3 of my sons. Each one has a card with his name, Mom and Dad's names, hotel we're staying at, medical information (2 of mine are allergic to bee stings). We laminated them so they'll stand up to water rides. They'll carry these in their own hip packs. I've also put PopPop's cell phone number on the cards. He'll be at home, but he's a familiar voice that might help them feel better if they're really scared.
Dad and I will each have another information card with us. It has each child's name, date of birth, height, weight and any identifying marks on it. On the back, we pasted a picture of each boy.
When you get to your hotel (if you're staying at a Disney hotel) or to the parks, talk to one of the cast members about how they handle lost children. There's one special place in each park where cast members take any children who are lost. But you know what? Not very many people ever need to use those places.
I hope these ideas will help you. It's okay to be scared. Just make sure you keep talking to Mom about it.
Good luck!
KangaT (I am SUCH a Mom!)