Some kids are just really wild by temperament and it shows up in their first year or two on the Earth.

Short of breaking their spirit there's not much the parents can do to make them less wild. Parents have to try to find a way to let the kid be wild and feel good about being a temperamentally wild kid while also recognizing that there are limits and boundaries. A similar experience is perhaps owning a dog that needs to run around outside every day.

But nothing in the autism or ADHD diagnoses says you can't learn social skills or that they shouldn't. Nor does it require anyone to endure abuse or physical attacks.

Assuming you're not the parent (since you say the more generic "family member"), probably the best thing you can do is try to befriend this kid and learn a bit about what's like to be them. And try to find traits in yourself that help you see why they act one way or another.

Another thing to keep in mind is that things you take for granted may be very distressing to this person. They may be able to hear noises or smell smells or taste flavors that you can't. The parents have a tough job because nobody wants their kid's preferences to dictate everything, but nobody wants to knowingly make their kid distressed when they can help it. That becomes a tough tradeoff when you have someone who is easily distressed by a world that isn't really designed with their brains in mind. This is where things like earplugs and sunglasses can help. It's a bit like being hungover all the time. If you reduce the sensory input everyone becomes less irritable and more even keel.

Having had personal experience with this problem, there is actually a simple but difficult solution to your problem:

Focus in the effect if the person's action on you, and set hard boundaries for yourself.

Note that boundaries are in this specific form:

"If you do X in Y situation, I will do Z"

The consequence to the instigating person (the Z part) is important. The person you described likely has a deficit in empathy, so feedback on actions has to happen to modify behavior.

Note that I'm not saying to hit back. Consequences are generally best when withdrawing something of value than adding something of negative value.

For example: "If we're watching a movie and you are physically rowdy, you will have to leave the living room and entertain yourself in your room".

Focua on how things affect you, and what actions you can take in response to boundary breaking behavior. Then be ruthlessly firm and consistent about it.

That's more like social anxiety.

I mean, the thing about autism is that your 'natural behaviours and feelings' can be things that others find deeply offputting, like hand-flapping or other strange repetitive movements or sounds, anxious skin-picking, avoiding eye contact etc. But not doing those things requires conscious effort, because they're natural to you.

So it's kind of like going through social interactions with an itch that you're constantly aware of but can't scratch. Or perhaps for a neurotypical person, imagine that you're instead not allowed to make any facial expressions or change in your tone of voice and you have to constantly monitor yourself to make sure you're not doing what is, to you, a normal and unconscious reaction. Of course everyone has to modulate their tone of voice and expression to some degree, but with autistics the gap between how they'd behave 'naturally' and 'what is considered socially acceptable' can be a much bigger one to bridge.

An analogy I’ve heard that I like is that the difference is akin to the difference between the “act” you put on for work, and the “act” you would put on for a high stakes job interview. Both are “just” normal proper business behavior but one is much more exhausting and if you had to do that later every day, all day, that would be a huge drain on you.

It’s a bit different though. If you have autism you might not even want to wish your friend or family member happy birthday. Whereas for neurotypical I think they would normally enjoy doing that.

Or you might not want to say hello or goodbye in the normal situations they are used. Or small talk. These are all things many neurotypical enjoy.

Yes.