Contact the BCU british canoe union . Most local kayak clubs do evenings for teaching kids , and outdoor centres may offer sessions .
Call the Kayak shop in Plymouth, Kayaks and paddles , they deal with all the local clubs and instructors
There is a club at Saltash that meets under the Tamar bridge on a thursday evening who may help .
Also in Plymouth I often see kids kayaking at Mountbatten ,which could be the local club Port of Plymouth CC or possibly the watersports centre .
There are also a few comercial outfits who take parties on kayak trips where you can go in a double with the kids .
tamar canoe expeditions is one such outfit who do an interesting trip up or down the river Tamar aimed at families and novices