Someone would sue a minimum-wage employee personally because they didn't say something to their kid playing dangerously? I know America is litigious but that sounds like it would never happen. They would sue the store owner if anything. The best thing would have been for the employee to keep their comments to themselves, which the snarky response probably reinforced.
Is this sue-happiness due to the high medical costs?
It seems the general idea in the US is that people are assumed to be idiots, so the burden is on companies to disclaim negligence or liability over accidents that might conceivably be unforeseen (by an idiot). Unless some kind of warning is placed on the cart itself (pointing out that it could tip over), it's not a stretch to think Home Depot might be found liable for a child hurting themselves in this situation[1]. All the carts in normal grocery stores are covered in similar warning signs.
Of course, some other countries tend to place a lot more confidence in the public's level of intelligence (and that's generally a good thing), but it can also be taken too far. The optimal legal landscape would probably use something in the middle of the "bubble-wrap everything" approach and the "jump at your own risk" approach.
[1] To answer your question "Would you personally sue them in such a case?": The answer is almost always "no". But those few rare cases where someone does sue can be incredibly costly for everyone involved. The only winners are the personal injury lawyers who often make money either way.
META: I know HN discourages commenting on voting, but there seems to be some odd downvote behavior going on that I haven't seen here before. I really can't imagine why this comment would get downvoted so much. To get this number of downvotes (let alone any votes this deep in a thread) in a short time smells of abusive behavior. I don't care about the points per se; it just makes me less inclined to contribute to discussions on HN when this stuff happens.
It's unfortunate that personal injury law firms make so much money by encouraging people to be "sue-happy" over dubious claims brought against companies that have little choice but to settle - not because they're doing anything seriously negligent (unless you subscribe to the bubble-wrap mentality), but because it costs less to settle and pay the plaintiff's fees than to properly defend the case in court.
I have no problem with the constitutional rights that allow these things to happen (that's not the unfortunate part). The truly unfortunate part is that the success of these types of legal threats has gone beyond just protecting the innocent; it has quite literally changed the public's mindset from one of seeking justice to one of seeking gain at the opportunity provided by an accident.
Imagine some kid falls and hits his head. The injury is so bad that the kid will be mentally disabled for the rest of his life and require full time care. You're a juror and you have the power to take away a couple of million dollars from a large company that'll make 11 billion that year in order to make sure this family's future is just a little less terrible. That this kid can get sometime of care and therapy so maybe he can talk again or just pay off some of the million dollars in medical bills they racked up on trying to save his life. I can see why it would be hard to side with Home Depot despite the fact they're not at fault.
But you're missing the point. People have to be cautious, because their employers require it. Don't be mean to people who don't control what they have to do.