I may be interested along with some/all of my kids, but it would just depend on the weekend and location. With our busy schedules the chances of it working out is probably slim to none.
Only piece of advice from being involved in a couple of get-togethers like this. Keep it as simple as possible. The more complex, the more problems it will create. My advice for what it is worth. LOL.
1. I agree with above in keeping it to a relatively calm stream as you are more likely to get more interest.
2. While overnight would be cool, that would likely limit participation since some wouldn't be able to do it schedule wise, and some may be apprehensive about camping over with relative strangers. This also requires packing more equipment on the kayak
3. As far as time of day, I would recommend having everyone meet at a set location say before 8AM. Whoever is there by that time goes, and if you aren't then you will miss it. Having to wait on someone who is late or possibly decided to blow it off stinks.
4. I would recommend doing something small like figuring on being to the destination at 3PM. This way you would only need to pack 1 meal and also would allow those who possibly drove a distance to get back at a decent hour. If some wish to go longer then they can plan for that on their own.