OK, here is what I've learned this morning poking around.
There's an additional standard for downhill bike helmets on top of the minimum CPSC standard all helmets must meet. ASTM F1952. I have not looked it up yet, much less read it with a critical eye.
DH helmets are likely to have the double D ring like a moto helmet. Or a Fidlock, a magnetic clasp, which are familiar to Euro moto riders but against the DOT rules here in the USA
In case anyone doesn't know, all of the full face helmets are intended for use with ski-type goggles. They might or might not interfere with glasses but usually you'd have to slide them in after putting the helmet on.
Enduro racing has been popular for the last few years. In enduro the downhills are timed, and the uphills are
not timed but must be done. This also makes for a popular category of bike, with medium-large travel but not awful to pedal. So, there are now lighter duty full face helmets and convertible ones with a removable chin bar. They have better ventilation by far than the DH-only helmets. They mostly still meet the DH helmet standard, which seems like something you should want. I feel as though if I had the removable chin bar, I'd probably leave it home, making it not really useful.
This review article is pretty thorough both for number of lids and how they were evaluated. Some of each style. You can see that the DH helmets are a lot more like dirt bike helmets and the enduro helmets are a lot more like MTB helmets
https://www.outdoorgearlab.com/topic...ownhill-helmet
All of the "top pick" helmets here are $300 and points north. Surely suffering from the same bike industry supply chain bla bla bla as well as pure inflation but still, expensive.
For kids I'm finding three categories. DH, BMX racing, and "Dad thinks it would be a good idea." The last category is maybe not up to that DH standard but they cost $100-200. The Kali offering makes the review pages because it's the lightest but stands out for having a token gossamer chin bar that looks like it wouldn't do much in a serious crash. But it might prevent those face injuries from near-0MPH kid crashes