Yes there are cameras at Walmart Self checkout registers to watch the customers for potential fraud or stealing of the items or whatever it is that they are trying to buy or steal.
The Walmart self checkouts also have someone standing near the self checkouts to oversee the self checkouts and they will also be alerted to any problems.
That also goes for if you're required to be a certain age to buy something through Walmart self checkout and if you're supposed to be a certain age the person will check your ID and then enter a password and confirm that you're 18 or over.
So you can't buy an age restricted item if you're underage through self checkout.