I lived in Ottawa for four years, actually, and my husband for over a decade -- but I haven't been back since 2013 and our kids have never visited. I'm actually really looking forward to revisiting some of my old stomping ground with them!
We haven't really filled out our itinerary yet, so I don't mind suggestions on things to do with the kids in particular (all elementary school age). So far we're thinking touring the Hill & Peace Tower, the Museum of Nature, whatever the Museum of Civilization is called these days, and maybe one of the river/canal boat tours.