As my research the same question on the internet, I got that the price for eyeglasses with frames starting was from $18 to $140 in 2009, and that they also charged $49 for the eye exam. The cheapest lenses are what is made of plastic. Its price was only $30. Walmart also provided the polycarbonate lenses. The charge for them was much higher than the former. Supposing you want to buy a pair of kid's eyeglasses at walmart now, the price must be higher than in 2009. What's more, for getting right glasses, I suggest you go there with your kid personally.
There are many kinds of the kids' glasses in the Walmart, and if you want to know the price for the kids, it will depend on what glasses do you want. If you just want to buy the sunglasses for kid, it will be a piece of cake. Then if you want to buy the prescription, I think it will cost you some time and money. At first, you should have your kids have eye exam so that the pros know more about the kids' eyes. Secondly, you will have to choose the lens and frames because the price of them is from $20 to several hundred. However, I advice you should choose the polycarbonate lens, which is safest for you. Hope I can help you.