Fun place for an 8 year olds Birthday Dinner
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.
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. :)