Montreal and Ontario

Here's the info on US - Canada border crossings. Used to be a valid driver's license was all that was needed. But if its a one time thing, an enhanced Driver's License may do the trick. It depends on where you're crossing the border.
Don't wait until the last minute. It takes a while to get one issued.

We stayed at Camping Alouette 20 minutes out of the city of Montreal....And that 20 minutes is not at rush hour which will easily double travel time. However, it's a nice RV park and one of the closest to the city. It's also on the highway to Quebec City which is a great city to visit....you could do a day trip....but it would be a long day. Not sure there's a lot to do in the area for kids.

http://www.parkbridge.com/properties/en-ca/Pages/home.aspx?pbCommunityId=CAL

x2 on the KOA in Niagara.
Just a couple of other suggestions for you:
1) if you go thru Montreal take AUT30 when you hit the city, it is a bypass and will reconnect with AUT20 on the other side of the city and avoids all the traffic, AUT20 will become 401 in Ontario and take you all the way to Toronto.
2) I took 2 different routes when we went out east last summer (via Ogdensburg NY and across US2 down and via Montreal on the return), and I much preferred crossing the Ogdensburg route. The drive across US2 is slower, but so much more scenic and it will also take you thru Adirondack National Park, Lake Placid and Mennonite country near Ogdensburg. The AUT20/401 drive is just flat and straight but quicker.
3) for either route (via Montreal or Ogdensburg) we stopped at the KOA in Mallorytown Ontario (just west of Ogdensburg and about 2 hours west of Montreal). It is an average KOA, but we really liked the hosts, so much that we would consider going back just for weekend trips.

KOA Niagara Falls is excellent and the city bus picks up at the front driveway for trips to the falls. Very convenient. The park has tons of activities for the kids. Also a short drive to the falls. Very easy.

Can't help you with Montreal but we stayed at the KOA Niagara Falls. A little pricey but close to the Falls and good for the kids. You could also try Bissell's Hideaway. 1/2 hour from the Falls but great place for the kids.

I should say we are pulling a 37 ft TT.