Hi Dianne!
I'm sure wandering children are a concern all parents have when visiting any crowded place. Disney doesn't rent any child locator devices for guests to use, however there are many on the market that may be what you're looking for.
When researching this question, I found that the Ion Kids Child Locator System got very good reviews from parents. It alerts you when your child begins to wander, and works like a gps. When you're outdoors, the signal can reach up to 350 feet. One unit can monitor up to 4 children at a time. Your child wears a wristag that looks like a watch, which is also waterproof. You can learn more about Ion Kids at their website -
http://www.ion-kids.com/.
Another option would be to rent a double stroller, which should allow both your children to sit comfortably and keep them from wandering away. You can also order safety tattoos from
http://www.safetytat.com/ that have your cell phone number listed, in case they should get lost.
Also, teach them to find a cast member if they should find themselves separated from you. Disney's cast members are very well trained in helping lost children and parents find each other. There is a system in place in all of the Disney parks to help in these situations.
My family loves doing the Disney Visa meet and greet in Epcot! It is such a wonderful chance to have one on one time with the characters, and it's always such a nice surprise to see which ones are in the room! You don't have to sign up ahead of time to enjoy this perk. Simply show up at Innoventions West in Epcot between 1:30 and 4:30 with your Disney Visa. One family at a time will be let in to meet the characters. A PhotoPass photographer will be on hand to snap photos, and you'll get a certificate for a free 5x7 photo that you can redeem later that day.
I hope this information helps! Have a great time!