I have two kids- 5 and 6 years old (4' tall) this summer. last year I made them greenland paddles to the same pattern 66"long by 2-3/4" wide blades. they go everywhere with them. the older boy tried a larger but still small Euro paddle and couldnt get anywhere.

My advice follow the greenland pattern with a little growing room. don't make it too big and don't make a peice of fine furniture. just use pine or other readily available cheap wood, and experiment.