The top face value for O tickets in $165 (including tax). So at $300 you must be dealing with a broker. Don't. Try booking directly through the hotel or at least via their website. Go to www.bellagio.com and click on the O Cirque du Soleil link on the home page. You can easily check ticket availability for any night you are in town. You will not pay extra. Also, Luxor is part of MGM-MIrage, which owns Bellagio, so someone at Luxor might be able to help you with tickets if you are uncomfortable using a credit card online.
Again, I would avoid tickets brokers unless you are desperate and there are absolutely no seats available otherwise. I really enjoyed O at $165 a ticket, but $300 is ridiculous. I doubt that you would be disappointed with $100 seats, so do not feel you need to pay top dollar. Just make sure they are not partial view (not sure if there are any of those at O but there are at other Cirque shows). I also would not go for front row seats (where you might get wet), as it seems you miss some of the spectacle, but there are those who will argue otherwise.
As for dining, I suggest eating at Bellagio. There is a wide array of choices from reasonable to extremely expensive and an equally wide array of cuisine types. Again, check the Bellagio website for dining options and see what you like. Bellagio offers a dinner and show package that is fairly priced if you like the restaurant and menu choices( which are limited on the package). Possible casual dining choices for a family with teens are Fix, Sensi, Noodles and Olives (my favorite of these).
There are restaurant reviews available by clicking on the Dining tab near the top of this page.