Fun place for an 8 year olds Birthday Dinner
You don't say what you like to eat or what she is interested in.
My daughter loved Benihana. (Her grandmother took her while my husband and I went to Samba.) She also liked NASCAR Cafe.
My daughter loves Food Network so any restaurant by a Food Network chef would be great for her.
Yolie's is a Brazilian steakhouse where they keep the meet coming off the grill. It's a fun experience, if you like meat. We have eaten both there and at Samba, though it has been several years. The difference was between being in a loud casino and not.
For my daughter, the most important stop in Las Vegas is the M&M store. :)
Maybe The Rainforest Cafe in the MGM http://www.mgmgrand.com/restaurants/rainforest-cafe.aspx or the Tournament of Kings dinner/show at the Excalibur http://www.excalibur.com/entertainmen.... Or take her to see a show such as The Lion King at Mandalay Bay or other show she might like and have a dinner at a restaurant near the venue that has her favorite food.