Apples and crab apples are fine. They will really enjoy them.
I’m going to tell you something that might scare you silly but it shouldn’t. That’s not the intent. Quite the opposite.
You’ll occasionally see apple seeds on the “do not feed to chickens” list. They contain a poison. Most fruit seeds do. Have you ever swallowed an apple seed? I did a lot when I was a kid eating them straight off the tree. They contain a poison, so why am I still alive? Dosage. They don’t contain enough poison to hurt you or your chickens. People see that they contain a poison and freak out. Practically anything you eat has something in it that can harm you if you eat enough of it.
If you make a bunch of apple jelly and have a huge quantity of seeds, maybe it’s best to not give a huge pile of those to your chickens, but even then they are highly unlikely to eat enough to harm themselves. But if you make an apple pie, go ahead and feed the peelings, cores, and seeds to your chickens. Even if you only have two or three chickens, that won’t be enough to harm them. When those crab apples get ripe, they will probably eat them right off the ground. No problems. They won’t eat enough to harm themselves, not nearly enough.