I've just found this thread, don't know if my perspective is any use at all but anyway.
I've held a full bike license for about 11 years (first two years restricted, but automatic clearance to ride anything after that) and have ridden throughout that period apart from about 3 years when I did not own a bike. (when ds2 was small). I rode every day for a few years, in all weathers, all conditions including the sort of rain you cannot see through, with an open face helmet, sliding around in deep water in the dark.
Stupid perhaps but it was that or no way home...and it was before I had children.
I'm an experienced rider and a good rider. In other people's opinion too.
But I would not take a child on the back of my bike.
Your DH is being foolish and a bully, to boot, by the sound of it - how dare he make you feel so scared and uncomfortable.
Is he normally like this? If so that's another issue. But anyway. He is foolish and stupid to consider doing this.
First off, he has not ridden for fourteen years. I stopped for three and when I started again I was a danger to myself and others - not through being ignorant, but because I was no longer used to riding a bike. Any bike. And this was after many years of riding before I took that break.
It took me literally months to become comfortable and confident on a bike again. It was awful. I could ride, and had no accidents but I did not feel secure and that is CRUCIAL to safe riding.
Your DH will not be confident or comfortable on a bike. End of.
Secondly, it is not one of those laid back ones like a goldwing with an 'armchair' for the pillion, it is a proper sports bike (if he takes the TT again) or even a low rider like a Harley, which has no protection for the pillion apart from how tightly they hold on.
Modifying the footpegs is insane. And will make very little difference and anyway you can't take two pillion passengers at once 
Having poorly fitting helmets is the difference between the person living or dying, quite often.
(sorry to be blunt)
and country roads are among the most dangerous circumstances in which to ride, because there is usually no visibility, plenty of blind corners, hedges, could be horses, farm machinery, and often speeding cars coming the other way. They are dangerous and I avoided them at all costs during my riding career (only just sold my bike - another baby on the way).
I hope all these arguments further reinforce your determination not to let him take your kids on the back this weekend. He is being an utter dick and if he wants a fight about it, tell him to talk to your biker friend on MN.
Please stick to your guns and don't be bullied into this. It's wrong, stupid and basically a disaster waiting to happen.