Bellagio has lot's of families staying there. You daughter can not loiter in the casino, but she can cut through with an adult (21+). She can dine with you at any restaurant in Las Vegas that does not have gaming (meaning mainly bars). She should be fine attending afternoon tea, but you may want to contact Petrossian to make sure.

She may want to visit a "real" castle at Excalibur and play some games in the underground arcade. The pool is very nice at the Bellagio. Town Square has Game Works. Be sure to check out the Bellagio Fountains. You might want to ask this same question on Tripadvisor.com in the Las Vegas Forum. I am one of about dozen Destination Expert's for Las Vegas on that forum and I am sure some people can help that have kids.

Safe and happy travels to Las Vegas!