Asking because I’ve got a two-year-old and a four-year-old and I’m starting to realize that some of the things I let slide in year one are now becoming full-on battles. Like I let the bedtime stuff get loose and now 8:30pm feels like a negotiation every single night. Would love to hear what other parents wish they had locked in earlier. What’s the one thing that made the biggest difference? 
Oh MasterHeight the bedtime thing is so real and I felt that in my soul 
We went through the exact same thing with our oldest.
The one boundary I wish we had set way earlier is the one ask rule. You ask for something once, you get an answer, and that’s the answer. No repeating it five times until a parent gives in just to stop the noise.
Sounds simple. It is not simple to enforce at first. The first week of holding firm on it was rough, lots of tears, lots of but whyyyy. But around day ten it just kind of clicked for our son and now he asks once and moves on.
The difference in daily stress levels was genuinely wild. Like our evenings went from chaos to almost calm. I’d go back and start that one on day one if I could 
Screen time before bed. Set it early, hold it firm, thank yourself for the next ten years.
I did not do this. Our four-year-old had zero wind-down routine and then we wondered why she was awake until 11pm wired like she’d had an espresso. Turns out the tablet light at 8pm was doing the exact thing you’d expect it to do if you knew anything about how sleep works. Which I did not. At the time.
Anyway. Screens off an hour before bed. Non-negotiable. Start it before they even know what negotiating is and your future self will be so grateful 
I think the one that gets overlooked most is emotional boundaries, specifically teaching kids early that not every feeling needs an immediate reaction from a parent.
What I mean is: when my daughter was little I would jump to fix every emotional moment. She was upset, I was solving it. She was frustrated, I was right there smoothing it over. Felt like good parenting at the time. The problem was she never really learned to sit with a feeling for even a few minutes on her own. Any discomfort meant mom needed to appear immediately and make it go away.
Setting a boundary around that, giving her space to feel something before I stepped in was genuinely hard for me at first because it felt like neglect when it was actually the opposite. It was teaching her that feelings aren’t emergencies.
Mine is simple: no food outside the kitchen. Full stop.
Started it late and spent two years finding crackers in places crackers should never be. Inside couch cushions. Under a bed. In a backpack pocket from eight months ago. Just crackers. Everywhere. Always. Set the boundary at age three and the house has been dramatically more livable since. Kids adapted within a week. Took me longer to believe it had actually worked than it took them to stop doing it.
@GlassTech what you said about emotional boundaries is something I think about a lot. We came to that one late too and it really does show up differently when kids are older.
The boundary I’d add to that is: parents are allowed to say I need a minute before responding.
Like modeling that for kids early, showing them that even adults don’t have to react instantly, that it’s ok to pause before you speak. My son started doing it around age six, just naturally. Someone would say something that upset him and he’d say I need a second before responding.
I cried a little bit the first time he did that not gonna lie 
Such a small thing that genuinely changes how kids handle conflict as they get older. Start it before they are school age if you can.
@MasterHeight you mentioned the bedtime negotiation thing and I just want you to know it does not get easier on its own. It gets worse. And then you fix it. And then you wonder why you didn’t fix it two years earlier.
Ask me how I know 
the boundary around talking back. Specifically there’s a difference between a kid expressing disagreement and a kid being disrespectful about it. You can absolutely say I don’t think that’s fair in a normal voice. You cannot roll your eyes, slam things, or yell about it.
We drew that line clearly when our kids were young and it meant that disagreements stayed conversations instead of turning into blow-ups. They still push back on things, they should, that’s health but the tone stays manageable.
Set it early and mean it. Kids read very quickly whether a boundary is real or decorative.
Ok real story
My son was three. We were at a restaurant, a sit-down restaurant, the kind with actual tablecloths and he decided that was the perfect time to test how loud he could scream for no reason. Just to see. Just as an experiment.
I had never set a clear expectation around public behavior. I’d sort of just hoped he’d figure it out by osmosis or something.
He did not figure it out by osmosis.
We left. Full plates on the table, apologized to the people around us, packed up and left. And on the way out I told him calmly why we were leaving and what would need to be different next time.
Three weeks later we tried again. He was perfect. Sat there eating his pasta like a tiny gentleman.
The boundary was: in public spaces we use inside voices and stay seated. I just should have said it out loud before the tablecloth incident instead of after 
Consistency itself is the boundary.
Not any specific rule. If you say a thing, you do the thing. Every time. Without exception based on how tired you are or how inconvenient the moment is.
Kids figure out very fast which boundaries are flexible. The ones you enforce 80% of the time? They will find that 20% and live in it.
The boundary I’d add is around your own time as a parent. Teaching kids early that mom or dad has moments that are not available, you are on a call, you are in the middle of something, you need ten minutes, and that those moments need to be respected.
My kids know that if my door is closed they knock and wait. We started that when they were tiny and it has paid off in ways I didn’t expect. They are more patient in general. They understand that other people have needs too. And I actually get to finish a thought occasionally which is a genuine luxury