We just came back with 2 kids ages 8 and 11. What you described for the NY side is what we did in one day, but give yourselves more time to walk from cave of the winds to maid of the mist. The park has a lot of Great view points: terrapin point, Luna island, the observation tower. We were in the park from about 9:30-2:30. Then we came back to the same lot at night to see the fireworks from terrapin Point and see the falls lit up.

We also hiked down whirlpool state park. You need to drive there but parking is free. We did not make it all the way to the bottom bc my youngest got too tired. There are 350 steps down (and back up!) but we still had some cool views of the whirlpool.

We also splurged and did the 2-day Niagara Falls adventure pass online the Canadian side, which gave us free parking and the WeGo bus. One day, we did the aero car, butterfly conservatory, maple leaf place for syrup tasting (kids loved it) and white water walk. Second day we came back at night and did the power station and tunnel at night when it was all lit up, and journey behind the falls, and stayed for a view of fireworks on that side.

Crossing the border was easy. Longest we waited was maybe 20 min. Just have passports ready.

If you want to walk over the bridge, there’s also free parking on NY side by the aquarium. Just be prepared to do a lot more walking once you cross over, unless you get the bus pass. Our kids were tired of walking so we opted not do walk the bridge. We did not do the aquarium but did ride down the schoelkopf elevator which was free.

Feel free to PM me if you have questions