I've made my choice! I appreciate all the tips and suggestions. I found a sevylor Tahiti hunt & fish kayak brand new on Craigslist for $60, so I bought it. I spent more than that on one rental, so I figured if I hated it, I could resell. I also borrowed my friend's fiberglass canoe and inflatable solstice voyager fishing boat.
The inflatables won. Kids hated the canoe. They would get excited about a turtle or fish and both would rush to one side to see it. The also like to take the net to try to catch turtles or minnows but the sides are higher and it's much less stable. I also did not like the weight and extra work portaging, and I had a hard time with tracking. It was a 3 person model, about 14' long I think.
The inflatables filled in minutes with the electric air mattress pump. Extremely stable, even with kids jumping side to side. You'd have to work pretty hard to flip them. They do drift a lot and catch wind, but were plenty easy enough to paddle. One of the best things is that they have lower sides so you can dangle your feet in the water.
They definitely aren't right for everyone or every situation, but they're roughly 10% of the cost of a hard boat, and I can carry both the kayak and fishing boat down to the ramp at the same time (I could barely manage the canoe solo I'm ashamed to admit!).
So far we've taken them in 2 local ponds and a small river, and the kids are already begging to go somewhere today. Wish I had done this sooner! We can't wait to take them camping!!