In response to:

But my kid is nice!
Why should I have to put her (him) on a leash?

Your kid is nice. He stays right next to you and has never bitten or pulled any dogs tails.

The problem is that there are other children that aren't as nice and obedient as your child. The owners of those children see you with your child off-leash and think that it is all right for them to have their children off-leash too. There is no way to rate the niceness of all children and allow just the nice ones to be off-leash. That's why we are lobbying to change the law so that ALL children have to be kept on leashes in public.

Besides, even the best behaved children can bite when surprised, or when they feel that they or their companion are threatened. Is it worth the risk?

Completely at ease with your child. You know what it is likely to do. Never saw your child before. No idea whether it is friendly or vicious.
Let your child pull your pant leg. Not thrilled about having their pants pulled by children, regardless of how benign you think this assault is.
Happy when your child screams and cries. Not happy to have children crying or screaming.
Keep your children clean. Not interested in being touched by children who don't know better than to play with dog waste.
Know that your child won't bite anyone. Has a right to expect you to be responsible and to keep your child on a leash, no matter how nice you think he (she) is.