Laws are not universal, and Casinos can set their own policy within the requirements of the applicable local laws.
Where I live, there are some small casinos and on a nearby native American reservation there is a huge casino (they're allowed to offer a lot of games that the off-reservation casinos can't have). These are stand-alone casinos where the only thing in the building is the gaming area, and by law you must be 21+ to even walk in the door. Most of them have security guards checking ID at the entrance, or at least someone watching surveillance cameras so they can immediately identify anyone who's age is questionable and go check their ID before they start playing (and kick them out if they're under 21)
On the other hand, if you go someplace like Las Vegas, many of the casinos are at hotels, and they're set up so that you have to walk through the casino areas just to get to the elevator to go up to the hotel rooms. Generally speaking you aren't going to have a problem if children walk through with their parents, as long as the kids don't stop to play any of the casino games.