Most casinos, not just in LV, are forced to allow minors to walk through the casino due to the design of the facility. Want to go to the bathroom, the only way to get there is by walking through the casino floor, the hope being you will drop a few $ in a slot machine on the way through. There are designated walkways (different color carpet) that she must stay on. She will not be allowed to stop, must keep moving. They will be watching closely if they discover she is underage. If they discover she is underage I'd suggest leaving that particular casino immediately, you might be able to get away with her watching at a different one.
Experienced this with my obviously underage children in a casino.