well it depends. If it is something I disagree with because it's not what I would do but it isn't going to cause the child immediate danger (like cocacola to kids for example) I say nothing. It's not my place. I might in conversation say how my kids aren't allowed to have it. I just got back from a week with my bestie and she lets her 2.5 year old have coke and I did tell her that its the worst one because it's caffiene, but I left it at that.
If it is something that is endangering the child like playing on the road (yes it happens in my street), not holding hands to cross the road (my sister and her 2 year old), running around the car when someone is getting ready to leave (my friends house) or leaving bubs in cars then I say it and give reasons WHY it shouldn't be done and in most cases I take action (getting the child and holding their hand, removing them from the road or calling the authorities when last week I saw a lady leave her bub in the car while she went running into the shop, I was on the phone to the police, they came within 5 minutes and they smashed her window and removed the child)

So I think it really depends on what the parenting is