He threw tequila on the kid. According to reports, some of it got in his eye. If you've never had alcohol put in your eye, I really don't recommend it.
When you're an adult, don't harm kids. Yes, that includes throwing alcoholic drinks on them. Also, don't be the type of adult who says "they'll be fine" about a kid. It is fucking scary to be a kid when an adult acts that irresponsible with you and you can never know what kind of an effect it will have going forward.
Good on AEW, Amanda Huber and the members of the audience who went out of their way to comfort the kid.
As for MJF, what he did was wildly irresponsible, immature, stupid and just plain shitty. He is very, very lucky that an adult who can handle themself and looking for a reason wasn't around, and there might come a time when that happens. Also, under California law, what he did was simple assault. Beyond that, it's another thing that goes to show that MJF really isn't mature enough for the spot that he's in. I get that being an edgelord is part of his schtick, but it's the very thing that drags down everything that he does. He's shown that he doesn't need it at all, and frankly, neither does AEW. What they need for him is to grow the fuck up.