Don't overlook visiting the Fallsview Casino just because your kids are with you. My son is 9 and has enjoyed visiting the casino complex with us a couple of times. Obviously, children cannot enter the actual casino itself, but there is quite a lot in the complex for a family to do.
The Doubletree is not that far from the casino, but if you do drive, park in the casino garage, then have you or your spouse go into the casino proper, visit the PAC desk, and upon showing identification, receive a free PAC (preferred access club?) card. That will entitle you to park for free (just show the card upon leaving the garage).
In the main lobby of the casino there is a huge futuristic fountain, very Orwellian in design. Every hour on the hour, starting at about 6 pm or 7 pm, there is a light and sound show at the fountain that your kids will love. Often, they will also have living "statues" on display in front of the fountain; you can have your photos taken with the young man or lady (tips appreciated, of course!)
There is a shopping arcade with some fun shops for the kids, like Turtle Pond toy store, and there is also a fudge and candy shop (very popular to watch the employees make the fudge and peanut brittle - free samples, too!)
There are quite a few restaurants in the complex. Johnny Rockets is a fun diner style restaurant with really great shakes and burgers. My son likes the ice cream from Pazzo Matto. The Grand Buffet is also really good at the casino; however, only adults are allowed there as you must pass through the casino to get to that restaurant.
If you visit on a Friday night, just before 9:00 pm, go down the escalator at the back of the shopping arcade and go right outside the doors to the falls overlook. Turn to the left, and voila! ... a beautiful fireworks display.
Just across the corner from the Fallsview Casino (I think at the corner of Fallsview Blvd and Murray Hill) is a two storey building housing Wolfgang Puck's on the main floor, and Cafe Tu Tu Tango on the second floor. I dined at Wolfgang Puck's last summer and really liked it. I haven't been to Cafe Tu Tu Tango yet, but I hear it's good, and I have also heard that they have art classes for kids on Sunday afternoons from 12 noon to 2 pm. This might be something you'd like to check out.
Good luck, and have a great trip!