Hiya, pal!
With kiddos that young, I can understand not wanting to freak them out about future plane trips.
Our oldest son is going to be six next month

and has seen some 9/11 footage on television at various points. We try and be as practical as possible, with a heavy dose of "parental guidance."
For example, he asked me what happened to the buildings after seeing video of them burning, and I kinda sorta hedged by stating that there was a fire in the towers. He then asked how many people were in those buildings and did they all get out (kid doesn't miss a trick in the realm of making ye olde father squirm, I tell ya), and I told him there were a lot of people and just like happens in some fires, some of them didn't survive

. He hasn't seen the collapse or the impacts of the jets or the more graphic footage of the victims in the WTC that day, and I will be very cautious when introducing such material to him.
Insofar as the Shanksville site is concerned, it all depends on how "honest" you want to be with them. Since it sounds like you are (probably correctly) concerned with having a negative effect on their feelings about future air travel, I would hedge here and explain it as a fire where a lot of people were, unfortunately, killed.
There isn't any way to explain a plane crash other than to use that very terminology which would be most likely to scare them (I guess in that respect I got off lucky by being able to confirm the obvious when questioned about the Twin Towers--yes, son, it's a fire).
I don't think I'd go so far as to call it an accident, though. It wasn't, and while a five year old may not need to know about terrorism and other nefarious world evils, neither do they need to be misled about it. No useful purpose would come at this age by telling them about "the bad people" who perpetrated the acts though, IMHO.
Of course, I am a professional historian with an intense interest in, of all things, the Titanic disaster in 1912 (and I was into it WAY before the movie...and the kids...and the wife, come to think of it...), so my eldest is kinda sorta used to seeing research materials/photographs/video of various aspects of that incident. Now I am NOT drawing ANY comparison between the Titanic and 9/11, other than both deal with subject matter that may be a bit daunting for many children (and their parents) to deal with, i.e. the untimely deaths of thousands of innocent victims.
When you stop to think about it, from a parental perspective and wanting to explain things as succinctly (yet as accurately) as possible, there actually IS a great deal of similarity between the two events, in that lots of people were killed. The whys and hows of their deaths aren't going to do anything to a five year old but scare them...if the fact of their deaths doesn't do that on its own.
Whew, this is long! My advice is to be honest, but be what I like to call "parentally honest." That is to say, be deceptive by omission (i.e. don't call Shanksville an accident, just as you don't want to call it a plane crash for fear of upsetting them regarding air travel--neither would comfort them, but the second explanation, at least, is true...there WAS a crash). Call it an incident, a fire (that's actually probably a lie in this case, but YOMV on that one), or better yet, if asked, state somberly that it was a tragedy in which a lot of innocent people died.
If they ask what a tragedy is, you can tell them it's an event that shouldn't have happened, but did...then breathe a sigh of relief that you got off so lucky with the explanation.
Of course, if they ask if there were little kids on the plane, you have a bit more of a sticky wicket to deal with. (ETA: Just checked, and there were no children aboard United 93; the youngest passenger was 20. Not to minimize the tragedy, but you know, as a parent, how kids are when other kids are injured or killed. It must have something to do with realization of their own mortality or something. But still --->) Same thing goes if they ask if the people were mommies and daddies, etc. It's thinking about them asking questions like this (and their probable reactions to the truthful, unvarnished answers) that make me want to go wake up my kids and hold them, and to tell you to lie and minimize the reality...but I won't tell you that, because it's not what I believe we should do when approaching this matter with our children. However, at the ages we're dealing with, there IS absolutely such a thing as too much information...and what would be the point or result, other than to scare them (unintentionally, of course, but you can't unring a bell).
And, at their age, obviously if they don't ask, then don't volunteer anything beyond that it's something that mommy and daddy want to stop and see. Then give 'em (and each other) a hug

and count your blessings.
Best Regards,
Scott in MO