Hi Sheila!
My two-year-old and four-year-old loved their MagicBands on our Walt Disney World vacation earlier this month. However, they did keep slipping off until I realized how to take off the outer edge for children or for adults with smaller wrists. MagicBands are adjustable, and there is a gray outer band that you can basically peel off. I just punched out the inner color and voilá - a smaller MagicBand. No tools needed.
Although my two-year-old didn't need a MagicBand, she still wanted to accessorize like her big sisters and sport it around the theme parks. Children can take off the MagicBand or pull it off if they want, so if you're worried about losing it, I would just check their wrists occasionally.
MagicBands are waterproof, too, so no need to take them off for swimming or hand washing.
If you have any other questions, please ask away! We hope to see you real soon. Have a magical vacation!
Tania