I decided to check mine out and ended up taking one apart since it was fairly simple to take apart and put back together. The album is below. My observations are as follows.
It doesn't seem that it can fall apart with the cover on, but the cover comes off fairly easily. The entire dis-assembly/re-assembly doesn't require any tools and can be done with bare hands. The key to holding everything together is the metal tension wire, and once that is taken out, the assembly easily comes apart (EDIT: with the cover off).
(disclaimer for the next statement, it is an unqualified opinion only)
The GOOD NEWS is that I see no way that this could fail during an accident, unless it has come apart before the accident happens. As soon as there is tension on the belt, the assembly is held together via the tension, the supporting crossbar needs to slide out opposite the tension.
Jump seat belt - Imgur
EDIT:
One more observation, the assembly does easily come apart without the cover on and tension wire taken out, but would need some slack to allow the bar to move back far enough to come out, so coming apart while being worn by a child seems hard, but I'm no expert here of course. So not saying anything one way or the other, but it seems like it may be (and hopefully is) a one-off defective part and not an overall design flaw.
EDIT 2:
I re-examined the seat belt assembly and will note one more observation. The tension wire does seat in very shallow indents in the crossbar. If the tension wire does slip out of the indent, it can slip down the side of the crossbar and while difficult, it is possible the crossbar can turn sideways and come out the side with the cover (so the cover would also have to be off). The album below shows the crossbar indent, the tension wire properly seated, and the tension wire slipped down the side. EDIT: My tension wire was not slipped down the side, I manually did that to show it can fit down the side of the crossbar.
Jump seat belts 2 - Imgur
EDIT 3:
For anyone concerned about the tension wire and want to check your own jump seats, you can easily see the wire and if it is seated through the opening on the connector. You do not need to take the cover off or anything apart. I just checked all mine in about 30 seconds with a flashlight to confirm they are all seated properly. This is what it looks like through the connector opening:
http://i.imgur.com/pGrIx1l.jpg
Would be interesting to know if anyone does have a tension wire unseated. Seems like it would be difficult for it to come unseated unless it was assembled incorrectly to begin with. Also, I have no way to know if the issue the OP had is in any way related to the tension wire, it may very well be completely unrelated.