You are most welcome to agree to disagree with me, but here is why I don't think Trelane is a Q:
But Trelane was a kid, and kids make stupid mistakes like that.
from
http://en.memory-alpha.org/wiki/Trelane
These clothes, along with his foppish mannerisms, were intended to suggest what he believed conditions on Earth to be at the time. In fact, he was viewing an image of Earth approximately four and a half centuries out of date, perhaps a consequence of the fact he was over nine hundred
light years away.
This suggests he was viewing Earth using a device, which led to the error:
If Trelane's powers hadn't fully developed, he may have used a machine to augment them.
Q's own son didn't need a machine to augment him in Voyager, and he was literally a newborn baby.
Trelane was playing, pretending he was some military commander. Trelane might not have been a name his parents gave him. I'm sure back in the 60s there were lots of kids playing with their toys calling themselves Captain Kirk or Mister Spock.
....
But the appearance as glowing orbs certainly fits with what Qs can do, and since they were there to get trelane to behave rather than interact with the humans, I don't see this as a particularly large problem.
I don't deny this is possible, in fact I even acknowledge that it is, but going by what we know on screen, it isn't the typical M.O. of the Q.
An simpler explanation, it seems to me, is he isn't a Q. Space is very big, and there are more than one Q-like species out there.
By your reasoning, it would also infer that the Organians (took the form of a light being), and the Metrons (took the form of a light being),could also be Q. Melkotians (displayed Q-like power, and set the stage for the Earp showdown), Apollo (Q like powers, used a device to augment himself), and even Sylvia and Korob (could be classified as adolescent Q since they use a device and didn't refer to themselves as Q), all displayed Q like power while calling themselves something else, but in the end, we know they are not Q, because they weren't written as Q in the first place, nor were there any retcons to say otherwise.
Basically, like some of these other races, the only two similarities between Trelane and Q, are their power levels, and the fact they wore costumes of past eras in front of humans. However, no where in TNG do they retroactively say Trelane was Q, and during TOS, obviously Q wasn't even conceived yet.
In fact, the only place where Trelane is stated to be a Q is, Q Squared by Peter David. And as I said, for those who read that book and consider it canon, they will disagree with me. But considering I don't consider the Trek books canon, and only consider what I see on screen as canon, I don't consider Trelane a Q. Just Q-like.