I did this yesterday with my two kids (10 and 8 yo). Agree with all the reviews that the whole beginning process is very easy - they give you life vests, demonstrate how to paddle, a few other tips, and you're off. You don't need to make a reservation - just show up. The woman recommended rowing up to Washington's Landing (40 min there, 20 min back).
Since I had two kids, my 8 yo had to sit in a "jump seat" that they put between the two main seats. He was a little cramped and the seat hurt his back. So, you can go with three people, but in the future I'll take just one kid at a time.
The paddling was harder than I thought, but I think that's because we weren't in sync. It would probably be easier in a single kayak.
Overall, great experience. Great way to see the city!