We take dinner with us and leave a stove at the bottom or pay for a shuttle service. When we get off the river we change them right away, and we try to end the trip at a park with a play area and picnic area. We get them fed and play a little more, then they fall asleep on the way home for the evening. Our biggest challenge was after we got off the river, and this system really smoothed out our trips.
This year I already got some new wet suits for them, and I keep extra clothes and towels in the raft in case they need to get warm and dry. We toss in some sand and pool toys for them to play with throughout the day. My son started at 3 and my girl started at 2.5, now they are 6 and 3.5 years old. My son is already talking about rafting this season.
We ended last season with some continuous class II -III, they did great. We only had one yucky weather day on probably 12 trips in 2013. Hope some of this helps.