That's pretty funny. Maybe he's craftier than you think though. I've always thought in hindsite I would have played my cards a little different, been more of a gentleman, nudge nudge, wink wink.

I'll keep working on my Formula Ford. I want to keep is as simple as Weight[pounds] = Height[feet] Squared, but that is a bit heavy. So that might be skin out weight, to allow for winter clothing, and the maximum pack weight might be a fraction of that. Perhaps 2/3 for a well seasoned little brute (I mean lady or gentleman that you need to slow down a little); 1/2 for the average child until you break them in (I mean introduce them to the joys of pack bearing); and 1/3 if it is a typical school pack without a hip belt.

So for Ford...
64" = 5.333 feet ---> 5.333 x 5.333 x 2/3 = 19 pounds pack weight

and for Margaret...
48 3/4" = 4.0625 feet --> 4.0625 x 4.0625 x 1/2 = 8 pounds pack weight

and for a 4'6" child with a pack without a hip belt...
54" = 4.5 feet --> 4.5 x 4.5 x 1/3 = 6.25 pounds pack weight.

Perhaps it makes sense to increase the fudge factor gradually from 1/3 to 1/2 up to 2/3 over successive hikes, in order to have time to get the child used to carrying that weight over distance, and to have time to prove the fit and stability of the pack also. I think I'll start Margaret at 4 pounds and work up. On a very good day she does 20km with no complaints and I don't want to change that.

Have you gotten Ford onto cross country skis or snow shoes?
That stuff seems poorly made in kid sizes also, but he is fair sized now.