(Note: I'm almost entirely self taught so there will probably be inaccuracies, so take the rest of this with a side of salt.)
I'm generally not very good at explaining/wording things, but whatever, might as well give this a try:
The first thing I want to mention is that I really hope that the faces aren't all exclusively side-views, since only drawing from a single angle probably won't help much.
I'll suggest starting out drawing side-views and front-views, and, once you're comfortable enough with those, move on to 3/4th views. You should then be able to move on to practicing more complex angles after that, though I can't say much about that since I've barely gotten that far myself.
In terms of anatomy, the only real issues I can find are the jaw and eyes (and maybe the cranium too).
The issue with the eyes is pretty simple: They aren't acting like they should from that perspective. From a side-view, eyes should look smushed, their width should be lessened while their height should stay the same.
The issue with the jaw is less apparent but it's still worth mentioning. The issue is most apparent in the first image as the angle of their jaw is a bit too steep and also curved a bit. It's also worth mentioning that the jaw starts a bit before the front of the neck, so the angle should seem like it's continuing from an earlier point, though the last two pics seem to get that just fine.
The cranium part isn't as important since it seems like that's something you're already aware of, but just in case you need to know; the craniums/skulls of the last two seem flat when they should be a bit more bulbous (If that makes any sense), a flat cranium isn't a physical impossibility since all that has to happen to make one is to leave a baby lying down all the time(Y'know since their skulls are soft and can easily be malformed if poorly cared for), though I'm not sure if you want your guys to have flat heads or not.
(This is literally me just guessing what their head shapes are, so it probably isn't accurate enough lol)
The only real advice I'll give after that is to make sure to use a circle as a base whenever you draw a head, you're probably already doing that due to how common of a tip it is, but it doesn't hurt to mention.
I'll leave an example of a head sketch below this, one for something more accurate to real life, and another more accurate to an anime-ish art-style since that's probably what you were going for with these.
It's also worth mentioning that these aren't really hard rules, they're there for the purpose of learning anatomy and whatnot. The only real reason you're told these rules is so that you can learn not only what they mean, but also how you can break them, as a stylistic choice usually.
Good luck, and don't take my rambles too seriously
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