I disagree with the idea of taking out the plane bit that's your money shot! It's the right sort of climax to a film about kids' imagination.
My word of advice would be: tighten. You should be able to chop a few frames off here and there without mucking up the pacing. And I'd take 30 off the kid's face the the very end.
That said, I think ligthouse_man was right, there are a few too many cuts.
Maybe see if you can drop a cut or two between 00:58 and 01:21. After the titles, I'd drop the first wide shot, and open on the MCU of the cowboy. Returning to the same wide shot jars. And I'd resist going back to the same shot when they're standing, ready to draw unless you're going make each return progressively shorter, building to a crescendo. If you don't give that sense of urgency and return to shots they've seen the audience will lose interest.
You've got the right idea with the mirroring shots of the two of them when they're walking to the stand-off, but I'd see if you've got any more (CU of feet pacing, etc.) and cut them a bit tighter to make the rhythm really taut. It'll make it look like they've done this a thousand times, and in the process more dangerous.
I like the distorted actor's faces you've got by getting the camera close at 02:51-03:15, but I'm not so keen on the noticeable swaying. There's probably nothing you can do about it now (motion tracking in Motion if you can be bothered?), but it might have been better to position the camera further back and zoom in so they don't appear to sway as much. You want it to feel like the slightest movement might get them killed. Along the same lines, tighter would have been better on the shots of fingers twitching over the gun. Subtler movements = more tense.
I liked the wide shots, very cinematic. The colours are nicely done, and the natural lighting looked great (I wish people would use it more). And the 'match cuts' were well done.
It was cute, the twist caught me by surprise. Don't want to say too much and spoil the surprise.