On one of the new amarican dad episodes (❗spoiler alert❗)

Roger opens a family friendly bar. Which made me wonder 'why do you have to be 21+ to enter the bar area of a restaurant I'm the United states?' It a ok to order drinks in the regular part of the restaurant, even when children are present. It's also not like there is a solid wall between the restaurant and the bar required, so children can still see what goes on on a bar. So, I don't really understand that regulation and would love to be educated.