It sucks and was 100% uncalled for and that customer is a jerk. But, at least, the barista wasn't at risk of getting burned as the drink in question was likely kids temp. I hate rude customers. Maybe he should have actually looked at the drink name.

Battery, actually, which is worse. Assault is essentially the threat or implication of violence where reasonable means to carry it out exists. Battery is any form of unwanted and/or destructive physical contact, by body or by proxy, where consent has been declined or can be reasonably assumed to be. OP, furthermore, not only does the fact that you were on the job have no mitigating effect on the penalties (unless your employer has some kind of bullshit waiver), quite the opposite; most courts have harsher penalties for people who attack and disrupt people where they work. Tl;Dr, that person attacked you physically; you could have their head in court.

They probably fired OP, considering how FMLs tend to go

In this case, it was likely a warm drink because it was for a four year old. But, still, he never should have thrown the drink at the barista.

That's more than disrespectful, it's assault.

I hope the guy was arrested.

A lot of them only say the name, especially if they're busy. It usually says the order on the side, either written or with a the ticket stuck to the side. I always check that, especially if they only say the names.

I'll always say "[drink] for [name]" even if we're busy.