Separate from technical legality is whether or not it really matters. If your kid wears the uniform, would that result in arrest and prosecution? Almost certainly not. Under our legal system, district attorneys have are given a wide latitude of what law violations to prosecute and which ones to ignore.
Sodomy is still illegal in many states. But, unless there are special circumstances involved, you will be hard-pressed to find a DA who would prosecute this offense.
Several years ago, there was long-haired-hippie-type living in our neighborhood, who's habit it was to (loudly) criticize the military. Any time you would see him, at any function, or event (or, just taking a walk), he would spew anti-military doctrine at anyone who was foolish enough to pause long enough to listen. As this was in a town where the majority of the population was active duty or retired military, you can imagine that he was not well-liked in the community.
Then one day, he started wearing an Army Field Jacket that he had obtained from a military surplus store. The jacket had all the adornments, including the "U.S. Army" tape, unit badges, a "Ranger Tab," and the grade insignia of a Staff Sergeant. Obviously, this did not set well with several members of the community. We contacted the police department, and even went so far as to print 10 USC, Sections 771 and 772 for them. The police consulted with the local district attorney, then told us that the DA's office had absolutely no interest in prosecuting the case. Therefore, the police department had absolutely no interest in arresting the individual, or citing him with a crime.
A few years later, I was working for an online computer internet company (CompuServe), as part of their online chat team. We had a frequent user there who said he was an O-6 (captain) Navy Test Pilot. This person had actually shown up at several Chat Events, wearing the uniform of a Naval Officer. I personally met him (twice), and had no reason to doubt him. He had extensive Navy knowledge, and talked the lingo almost perfectly.
Imagine my surprise when I later learned that this person was not in the Navy -- in fact, he was a Canadian citizen (in the U.S. illegally), and had never served in the United States Military. When he was caught (in the act of wearing the uniform, on a Naval installation), he was prosecuted (and given a prison term) for violation of 10 USC 771.
In the first case, the prosecutor had no interest in pursuing criminal charges. In the second case, the prosecutor was more than happy to pursue the case to the maximum extent of the law.
But, what about the military services? Do they care if civilians wear the uniform or parts of the uniform, and might they be willing to pursuade a DA to prosecute? It seems so. Some of the services have gone out of their way to include restrictions in their dress and appearance regulations (which are not enforceable against civilians, but tends to show that service's view on the subject). Army Regualtion 670-1, paragraph 1-4 states:
d. In accordance with chapter 45, section 771, title 10, United States Code (10 USC 771), no person except a member of the U.S. Army may wear the uniform, or a distinctive part of the uniform of the U.S. Army unless otherwise authorized by law. Additionally, no person except a member of the U.S. Army may wear a uniform, any part of which is similar to a distinctive part of the U.S. Army uniform. This includes the distinctive uniforms and uniform items listed in paragraph 112 of this regulation. Paragraph 112 goes on to define "Distinctive uniforms and uniform items:"
a. The following uniform items are distinctive and will not be sold to or worn by unauthorized personnel:
This indicates that the Army would not be very happy if they learned that a civilian was wearing one of the items listed above.
So, is your kid (big or small) going to be arrested and sent to jail for wearing a military uniform on Halloween? Stay away from "distinctive" items such as insignia, badges, and tabs, and I'll bet you three bags of left-over Halloween candy that the answer would be "no."