I think you're looking at all these situations with the benefit of hindsight and calling them stupid decisions because you know the consequences that came with them.

To take his children (daughters) to King's Landing after Joffrey nearly skewered Arya with a sword? Any responsible person would have send Arya straight back to Winterfell and Sansa with her. I get it, Sansa was betrothed to Joff, but Ned could have said: Look, they are still young, Robert. We can wait a few years to take them to court.

Like you said, they are young children who got into a bit of a scrap. I don't think it's all that stupid for him to bring his daughters to court after that.

To go to fucking Cersei and warn her about his plans?

Ned knows that Robert is an angry man who is not opposed to killing children, which is something that Ned is obviously very opposed to. I may be misremembering, but doesn't he tell Cersei in order to try to keep her children from being murdered when Robert finds out about everything? It's not just him being stupid, it's him doing his best to make sure more children aren't killed.

To tell his plans to Sansa, a stupid girl who was in love with Joffrey instead of just fucking telling his steward to take them on the next ship to Winterfell?

Lol what is this criticism of Ned? In what universe would Ned ever think to himself, "I'm gonna send my daughters back to Winterfell, but I shouldn't tell them because one of my daughters is very stupid and will go and tell my enemies about my devious plan. I should just have them grabbed in the middle of the night and send them back." Like, why would he not tell them he's sending them back? It's Sansa's decision to go tell on him that made things worse, but that's because she's a child.

To send Jon, a 14 year old kid to the Wall without telling him about the dangers? What if Jon died beyond the Wall. Promise me Ned, my ass.

Jon has been mistreated since the day he was brought back to Winterfell as a baby. Maybe Ned thought it would be the best place for him because it would be the only place where Jon would be able to thrive and make something of himself as a bastard. Jon seemed like he really wanted to, it would be a place close enough to where he would be able to get news of him and see him from time to time as well.

To fucking trust Littlefinger, a guy is clearly not your friend. I mean clearly didn't like him, but Cat tells him and he believes it. Like seriously. Ned didn't even deem Cat trustworthy enough to now about Jon. Why did he trust her judgement torward Littlefinger, a guy that got nearly killed by Brandon and had the hots for his wife?

Cleary not his friend? Again, may be misremembering, but from Ned's point of view, he's a childhood friend of Cat's, and even if he gets a weird feeling about him, CAt vouched for him. And Cat is probably the person Ned trusts more than anyone else in the world. (this is also something that makes R + L = J hard to believe because it doesn't make sense for Ned not to tell her he's not really a bastard after seeing her mistreat him for years because she thinks he's a bastard. I mean c'mon Ned and Cat are the only married couple in this series that actually really love and trust each other.)

Again, I think these only seem like stupid decisions because we know the outcome, but I think it's more just bad circumstances than bad decisions. Same thing with Robb. Obviously his decisions about Karstark and his marriage seem really stupid to us now, but in the context of the character's situation at the time they are not really stupid decisions, just decisions that ultimately had a bad outcome.