We took a family trip to Niagara Falls last year when my son was 4. He loved it! For us, it was part of a longer trip with several stops, so we only spent one full day there, which - in my opinion - was enough. It's VERY touristy, but if you're prepared for that and take it for what it is, it's a lot of fun.
The answers to many of your questions will surely depend on your children and their own abilities and whatnot, but I'll do my best to help!
The main tourist "strip" of Niagara Falls is on a slope, so I personally would favor the Beco over a stroller. Also, there are a lot of souvenir shops and "Ripley's Believe It Or Not"-type places that are fairly crowded and might not be too stroller friendly. I don't know if strollers are allowed on the boat tours or any of those types of attractions but I would guess maybe not?
My son didn't have a problem walking from our hotel down to the Falls by himself. We may have had to carry him part of the way back (uphill), but we had been in the car for 8 hours the day before and he probably was tired that day. Again, it depends on your child. It will also depend on how much walking you do when you get down there, i.e. are you planning to do the boat tour, or walk the paths under the falls?
Since we were only there for a day, we chose just a couple of attractions to do - we did the Maid of the Mist boat ride and the Sky Wheel. I wasn't sure if my little one would like the boat ride, but he LOVED it! It's very calm until they drive you right into the middle of the Falls, and then it's a little bit tippy and VERY wet. He was practically screaming with excitement.

Definitely recommended. The Sky Wheel was cool too - just a giant Ferris wheel basically, but you can see the Falls from the top. We did that the first night and the boat ride the day after.
Have a great time!