I have an 11 year old who's been on the water since he was 18 mos and an almost 2 year old whose first trip was 5 days on the Juan at 6 weeks old.
With that kind of age span, this is what I've learned:
1. Lots of snacks- our kids will boat hop until they've eaten everyone out of snacks.
2. The right kind of toys- things they don't usually use at home are good as well as favorites like stuffed animals for comfort. We like kites and bubbles since they make the most of a headwind. Glow in the dark or light up balls are fun for games of pass down the flat water.
3. Lots of snacks- the rule is that your kids will rather eat what other people brought than what you brought, so plan on trades.
4. Shade- a must have, especially for little ones who can't swim solo yet.
5. Lots of snacks- they will eat at least 3 times as much as they do at home, especially if they are boys.
6. Water- drink it, swim in it, splash them with it, drink more of it. DRINK MORE WATER!
7. A well-organized boat- Know that the diapers, extra socks, wipes, spare PFD, and snacks are easily accessible throughout the day. Plan a "play area"- for the littlest littles this is the footwell lined with dry bags; for bigger littles, this is a pile of gear in the back, a deck up front or a SUP or IK.
8. Snacks- nothing fixes the problem like food, especially a new and unique flavor or treat.
I think the most important thing I've learned is that they may not love it as much as you do every time and that has to be OK. Don't be heartbroken if they don't love it the first time out. They will either grow into it, or they won't, but either way, someday they will appreciate the effort you made to give them a unique experience. SYOTR, Kjirsten
"There is NOTHING--absolute nothing--half so much worth doing as simply messing about in boats. Look here! If you've really nothing else on hand this morning, supposing we drop down the river together, and have a long day of it?" -Wind in the Willows