My first boat was a military surplus inflatable raft from a mail order catalog. We got two paddles with it and my dad made them into a single kayak paddle. I went all over the Cross Bayou in Florida with that boat chasing alligators, snakes, and fish. My sons got a kayak, a pirogue, and an inflatable as their first boats. Today there are a lot more options for a child's first boat.
I still think the first boat should come at the age of about 6 years old. At this stage children just play with the boats more than paddle them any real distance. A boat is a source of self esteem and independence for a child that they should not miss. Kayaks are ideal first boats and now they make more child's sizes and models than ever!
Jackson Kayak is really bringing up the rec boat and whitewater market for children!
I really like Tim Niemeyer's "No Child Left Inside" initiative. You can read more about it on his excellent site here: Kids Wave Kayak
This may be the best thing you can buy any child and it is available all over the place just type "kids wave kayak" into your browser.
This boat is rated for up to 120 pounds and they are stackable! I think these may be the ideal solution for a young paddling family. Children are rarely interested in tripping but they like to play on the water. so a couple of these and a canoe or pair of kayaks for the parents would be great for camping near the lake or the sound. Once they little ones get bigger they could try them in the surf, but make certain is is really easy surf and they are properly equipped for it.
I think inflatable boats are actually the best solution for the surf because they don't hurt as much as you learn the lessons needed to stay out of the way of the boat. Hard boats in the surf can really hurt children so be mindful of what can happen. The problem with inflatables for children is that most of them are so wide the child can not reach the sides easily. I found the best solution here is two children per inflatable with one paddling on each side with single blades. Sometimes they get it right and it is a joy to watch. When they get it wrong it is usually hilarious! A parent wearing swim fins can be a lot of help in guiding them out through the surf and coaching them onto the waves. Ideally another adult is onshore with a good camera. I do regret not having a good water proof camera for taking pictures when my boys were young.