@Jeep,
At first was not sure where to put the wheel, saw most where to the middle and some to the rear. I built the frame then bolted it to the bike, then I cut a piece of wood the same height as the wheel and propped it under the frame where I thought the wheel should be. Then I placed weight on the frame where I though the boys would sit and kept adjusting till the frame flexed the least. Not very sientific, but it worked.
Think I'll upgrade to bolting it on, hose clamps were a nice idea, but I think a little weak with a lot of use they might brake.
My mate came round and took his kids for a spin, flat taps down the road so it works great.
The only design flaw is that the bar that sits above the knees is a bit low and they have to bend there legs to the side to get in. The car is still painted with primer so after the weekend I'l cut it and move it out a bit.
Pic of my two.
Pic of my mates to little ones