It may seem strange to teach kids to kayak while still on land but it is very important. The first kayaking lesson should always take place before you get into the water. For younger kids, this will serve to get them comfortable and prepare them for what they are about to experience. For older kids and teens, this is helps to maintain their attention while you still can.
Once in the water, it is too late to teach certain things about kayaking. Have the child sit in the kayak on land and explain the basics about how it feels to be in a kayak. This is a technique I use with adults also as there is often a healthy apprehension about flipping over. Sitting in the kayak while on land helps alleviate that fear. For older children who will be paddling on their own, I also teach them how to hold the paddle and the basics of the forward stroke before they get into the water.
As my son is only two, there is really no point in going through the kayak forward stroke while on land. I tell him things like not to stand up nor is he to lean over the side of the kayak. This is also the point where you will see what the best way to situate your child in the kayak with you is. For kayaks with small cockpits you’ll have to see if you can fit with your child on your lap