We love Niagara Falls, and generally go at least once a year.
If your budget can swing it, Great Wolf Lodge is awesome for kids the ages of your children. In addition to the indoor waterpark, the outdoor pool would also be open, and there's mini golf, an arcade, a bowling alley, Magic Quest and other activities right on site. Summer is pricey, though....but that may be offset by the fact that your dollar will far based on the current exchange rates.
The Hornblower Cruises (formerly Maid of the Mist), the Butterfly Conservatory, Bird Kingdom, Greg Frewin's show, the SkyWheel, and Cave of the Winds (that one is on the US side) are all very worthwhile attractions. There is also another indoor waterpark (Fallsview) right down by the Falls, and unlike Great Wolf, its open to the general public (not just hotel guests).
On the US side, there's a place called the Silo that is nice to visit in the summer. It was featured some years back on Man v. Food, and the outdoor seating area offers a lovely view of the Niagara River.
There are fireworks several times a week (free).
If you have never been before, I say give it a go. There's TONS to do (just be selective.....not all tourist attractions are worth the price of admission) and its very family friendly.