Happy birthday to your daughter. Twenty-one is a good age to turn when you're in Las Vegas.
A valid driver's license is the only proof of age she'll need.
As far as being carded, we know of no "independent certification symbol" that she can wear as proof. You'll simply have to be prepared for her to get questioned frequently about her age.
But hey, why not have some fun around it? For example, dress her up so she looks older. Or bet on how many times a day she'll be asked for ID. Or simply make sure her driver's license is at the ready, so she can flash it at anyone who might ask, before being asked. If you make a game out of a slight inconvenience that's inevitable, it might cut down on the annoyance factor.
Because it is inevitable that she'll be carded. Las Vegas casinos remain highly sensitive to underage gamblers (and drinkers and smokers), an attitude left over from the family-friendly '90s.
