Whoa, it is really a happy thing that you have got a fashionable kid. So, indeed, you can easily find vintage cat eye glasses for adults in the market, but rarely for kids. In fact, the difference between adults' and children's is the size of the frame. So, if you have the chance , you should search through the internet and see if you can find them, Ebay and Amazon are good options. But, if you cannot, you can just choose the smallest size for men and that would be ok.
If the eyeglasses have the vintage cat eye glasses for adult, there must be the ones for the kids. And you can search it from the internet. And buying one online will save you a lot of money and energy. In addition, you will have more choice for your kids.