I was worried about losing DS (2.5) at WDW this past summer, but really, once we got there, those fears were unfounded. We also use the hold a hand or ride rule which worked pretty well. But, if there was an open enough space, we would let him walk ahead a bit. The only time I was ever worried while we were there was during the Incredibles Dance Party in MK. He was so excited to dance with the characters that he just slipped through the crowd to be near them. However, between DH and I we were able to keep an eye on him and he was just fine. He even did well with sitting with the kids at turtle talk and the other performers at Enchanted Tales. Disney CMs really do a great job with little kids, regardless of their language skills. Even if he would get lost, the CMs are well versed in how to handle it. So try to relax and have fun. Remember, kids feed off your anxiety.
As far as IDs, we just trusted the Magic band. DS was thrilled with wearing it and wouldn't even take it off to sleep at night (lol). So, if he had gotten lost, a CM would have been able to scan that band and at least get his name. I am not 100 percent sure what all is stored on 3 and under bands as far as reservation or adults contact info, but I am betting that if they really couldn't find you, they would be able to get what they needed through the band. Also, familiarize yourself with CM procedures for lost children. They are trained to look in the area for a short time and then take the child to the baby care center in the park until a parent can be found. If you have been looking for longer than a few minutes, have a CM radio to baby care and see if your child is there. I truly believe no one knows how to handle lost little kids better than Disney. They will take care of you and your family. But also know, with 2 adults and 1 kiddo, the chances of getting separated at all are pretty slim, especially if it isn't usually an issue anywhere else (grocery store, mall, zoo, etc.)