Question about Child Harnesses/Leashes

[jorgies5]

Next month we are going to spend a week in New York City. I really need to get something like a leash for my 6-year-old. She is very impulsive, independent, quick as lightening, doesn't listen to instructions, and wanders off in public all the time. I have found several harness-type backpacks online, but they look like they are made for smaller children (18 mos - 4 years old). Does anyone have one that they think would work for a 6 year old? If so, what brand is it and where did you buy it? I will also be getting one for my 4-year-old. Has anyone seen a duo-type leash that is made for twins? Any suggestions would be greatly appreciated!