Pieced together from reputable news sources and alleged eyewitness reports...
The general consensus is that it was on the last section of the queue before entering the building, which is a raised walkway rather than a bridge. The fence here is a wooden 'picket' style, so you would expect each bit of wood to be held on with at least two but probably four screws or nails, which makes it seem odd that it'd just give way.
So if the young girl leant on a bit of the fence and it fell off, that would create a gap with a width of about 12 inches, not a huge amount of space to fall through but seemingly enough? In any case, there would still be the 'crossbeams' along the top and bottom of the fence, and the upright bits of wood on either side of the hole, to grab on to, not to mention the parents' hand.
If a fence panel was loose, the ride operator should have found it during the pre-opening checks. So if it was noticed but ignored, Chessington are in trouble. But if it was kicked or otherwise vandalised between opening and the time of the incident, it's unlikely a staff member would have been into the queueline so they wouldn't have known. There are conflicting reports about this, but it does beg the question, if eyewitnesses had already seen a loose or missing panel, why did they not inform a staff member, or if they did, why was nothing done about it.
But speculation will get us nowhere, all we can do is wait for the results of Chessington's investigation and the HSE's report on the incident, if they're made public, which they probably should be given the high profile this case has attracted. Until proven otherwise, I'm leaning towards the belief that this was an unforeseeable accident and no one is to blame.