Add zippers? 

Been a while since I had my newer Jayco (about 7 years ago).

Looking at the 2008 manual (page 64) there is Velcro on the bunk ends & corners- this could easily be adjusted by adding zippers or snap button or enhanced with a a few grommets and some kind of hook. Mostly I feel would be overkill.

My canvas on my 1979 Coleman is all bungee/shock cords on hooks and yet to have anyone even remotely fall out. I have slept (all 250 lbs of me) right on the edge and still holds.

As long as the bunk end metal hoops are taunt and the beds extended/locked all the way out- there is little to no give in the canvas.  Laying on the side wall on canvas and Velcro would distribute the weight quite well. Just try pulling off the Velcro from the middle of the canvas - say 4 inches away from the Velcro edge - force would be distributions across 6 to 12 inches at least.  You need several hundreds of pounds to pull it apart. Normally you have to pull - STARTING at one end and the force to pull apart would be concentrated as a very small section (about 1/4 to 1 square inch area only).

The rated straights are: Two square inches of Velcro are strong enough to support around 175 pounds (when evenly distributed). Velcro helps hold together the astronauts' suits and equipment.  NOW Thats strong!

I think a 50 lb kid - or even a 100+lbs teenager will not "fall" out of your trailer when properly fastened. However if you FORGET to press the Velcro together then falling out is VERY possible.  I always to a double walk around to make sure all flaps/straps/zipper/velcro  is done up.

(If they EVER had anyone even "nearly" "fall" out they would have been sued and issued recalls LONG time ago.)

Come down to - ensure it setup correctly and everything is in good shape! Have the spouse/partner/other adult double check all is secure - then all is good.

NOTE: if you have Velcro HOOKS fill up with junk (threads/fluff/fibers etc) and does not stick very well anymore - get a fine wire "dog" or "Slicker" brush and use that to clean out the hook side! Would work like NEW after!

We have velcro & bungee cords. Hasn't been an issue.
On our previous pop up, though, it was just tension and bungees and our puppy fell out in the middle of the night on his first camping trip. He wasn't hurt but did have a very surprised look on his face when I opened the door  :o.
I do like the permanently attached canvas that some have shown on the newer HTT's and I've been thinking about how to modify ours. I think that when the eliminated the extra rain flap layer that was a mistake. Water can just run down the canvas and into the plastic track where the canvas attaches which can lead to water damage.

To "restore" Velcro to full working strength use a fine wire "dog" or "Slicker" brush and use that to clean out the hook side!  (Not the softer loop side.) Would work like NEW after! Make sure all Velcro is securely attached to both sides and pressed together.

Zippers or other physical or solid attachments is your only other option. Velcro is very strong and providing your canvas is put up properly and taunt - there should never be an issue.