Someone will probably say that's not economically viable. Besides, sport in the USA is almost invariably a family affair. So for every few kids in the crowd you'd likely have a parent or guardian watching over them whose cash would count. Peaking the interest of more children of course would also broaden the potential for merchandising. As any parent in Europe will know, soccer strips are on sale everywhere.
Kids like to wear their club colors.
The gear doesn't come cheap. All of which equals more revenue. I know the MLS has its gear available at its website, and I'm sure it's on sale at the stadiums, but try finding an MLS kit in a mall. In my experience, it can't be done. And by missing out on the casual shopper I think the MLS is selling itself short.