When it comes to how to talk to each other, there are a lot of invisible lines. The tricky part is finding out where they are and trying not to cross them. They tend to move based on who you're talking to. Some people don't care if you're blunt and opinionated while others find that style annoying and obnoxious. My rule of thumb for any communication that could be offensive is "If it's offensive to some, then it's offensive to all." Therefore, refrain. Everybody's not digging what you're saying.
So how do you find the lines? Actually, it's up to other people to draw the lines for you since there are variations. Here are examples: When it's clear to you that someone doesn't like coarse language or they don't like to talk about politics, religion, or money, then those are the lines you shouldn't cross. When a co-worker doesn't mind personal jokes about himself, but he stops you at making jokes about his family, then the lines have been drawn. Likewise, you should draw lines for others about what you find tolerable and what you don't. For example, I won't be disrespected by people who like to call females derogatory names, who ridicule my religious beliefs, and who waste my time with idle chatter at important times. Where are your lines? How do you establish them?
The short answer is when you feel like you're being disrespected or taken advantage of by someone in something they say and do intentionally and repetitively, then it is up to you to bring it to a halt. Stop trying to accommodate people who are inconveniencing and hurting you. You don't owe them anything. What you owe yourself is peace of mind. You can't find it if you cringe every time somebody is doing something that you find disturbing or irritating.
So how do you set parameters? You simply say it. It might not be comfortable, and you may be one of those people who is conflict averse so you choose not to say anything. But the only way you set boundaries is to let others know where you've drawn the lines. You don't have to be rude or mean, but you should be firm. It could sound like: "Hey, I would appreciate it if you didn't talk to my kids like that." Or, "I don't mind stepping in to help you out when your workload piles up, but I can't keep doing it this frequently."
Set your limits, and don't feel bad about it. Draw your lines and don't move them. Don't compromise your preferences because people keep bumping up against them. Be patient but remain strong.