It would not bother me, I have on a few occasions had people stop to take my kids pictures, but the person almost always asked. We were at a medieval fair thing, and we had people stop to take the girls pictures in there princess clothes, at central park we had someone stop to tell us what a beautiful smile my daughter had and could be please take her picture, and just recently somewhere we went someone stopped to take there picture.
As I think about it everyone has asked me first, and I am not a sky is falling, worst case scenario person, so it does not bother me. But, like I said in my case everyone did ask, but I still don't think it would really bother me if they did not.
When we were at animal kingdom last year, there was this little boy, about 2, who was dressed to look like donald, yellow rain boots, white shorts, the shirt, and hat, he was the cutest thing I have EVER EVER seen, during the parade all the people in the parade were coming over to him, the CMs who were by the donald float made sure donald saw him, he was the cutest little thing I ever saw. I took his picture, and it is one of my favorites, it captures the joy we had at that moment.
But, in the end they are your kids, you have to do what you feel is comfortable, and if it creeps you out, then you have every right to ask them to stop.