Yeah probably wont do it unless they are like 16. 12 is a little young to be wondering around on their own. I get that they go on field trips, but even then they have chaperons close by. Our 8th grade field trip was over night in Louisville and we were always in groups, BUT we also had an adult w/ us all the time...and we were over 12.
Many middle schools let the kids wander in theme parks without adults with them. When I was 16, I was going into NYC with friends (and it wasn't even close to being as safe as it is now). I see your kids are very young. I don't know many 12 year olds who would not be able to be at WDW for a few hours without adult supervision.
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I do like to think we are instilling a love of adventure too though. We just gave our oldest son the gift of skydiving for his 19th. He loved it. He has had many other adventures growing up as the only boy grandchild of my Dad's for 7 years 
little flower princess will be my most adventurous of all. There isn't a thing my boys do that she will not attempt. She is speech delayed and had some motor delay because of a condition she was born with but she is full speed ahead now. This kid didn't walk until she was 18 mths old - now she roller skates, swims like a fish and will pretty much do whatever we will let her do.