The thing about wisdom is that it requires understanding, which is different from knowing. I can know a rule without understanding the motivation or underlying reasoning of the rule, and crucially, I can still follow the rule even if I don't understand it. This is useful for children because there are a ton of things they don't understand about the world that can hurt them if they're not prepared for it. Take for example the rule: "look both ways before crossing the street". It's a rule that is taught to children because running into a street without looking out for cars can get kids killed. There are a million situations like this. Not all of them are as starkly life and death, but kids who have a good set of rules can move through the world more safely and comfortably than kids without, and they can learn rules faster than they can learn the underlying logic of them. That can and should come later.

There are also rules that aren't so good. I think "make sure you finish your plate no matter what" is a rule that arose from a well-meaning attempt to prevent food waste, but it's a rough fit for the purpose and it can make it hard for children to develop their own understanding of their eating preferences. I wouldn't give a child that rule, I would spend a little more effort coming up with a solution to make sure kids are getting the nutrition they need without resorting to such ham-fisted measures.

Screen time rules are tricky, because we haven't had constant access to screens for very long, and the way we interact with them (TVs to PCs to smartphones, to say nothing of the media landscape in which these devices are situated) is still changing rapidly. Parents are doing their best to keep up and it is overwhelming because nobody really knows what the best rule is. I think it's easy to say "just do whatever's best for your child" if you don't have kids because it's hard to grasp the true scope of the problem of parenting: there are simply too many threats or possible threats to a child's well-being to individually tailor solutions to everything, and you as a parent, have to pick which areas require tailoring and which areas have sufficient rules to fall back on.

I don't think this is unique to children either. There are plenty of domains of knowledge where knowing a few good rules can make life a lot better even if you don't understand the mechanisms behind them. I don't drink pond water even though I am neither a microbiologist nor a public health expert. I wash veggies before I eat them, and I wash my hands before I eat. I take medications exactly as my doctor tells me to and I consult with them before changing things.

You may be thinking at this point, "but I understand all of those rules, why should I follow rules I don't understand?". Fair question. For children especially, but also for humans in general, I think the knowing of a rule has to come before the understanding of the rule. Understanding is where nuance comes in. You know the saying "there's an exception to every rule"? There are some people who take this to mean that rules can be shrugged off when inconvenient. These people are, largely, fools. The exceptions to rules can only really be identified when the rule and it's motivation are clearly understood. Sometimes when I'm doing emergency repairs on a software system, I will ignore the usual rules about code merges in prod. This has backfired on me before, because I did not take the proper alternative precautions one should take when moving quickly, but most of the time I think my handling of exceptional cases has been pretty good, because I have a good understanding of why those rules exist, the problems they're supposed to prevent, and when they can be safely(ish) broken.

Which brings me to my final point about wisdom, or understanding. Knowing about a rule, or fact, or domain of knowledge can feel an awful lot like understanding. Most humans suffer under this delusion, in most areas of their lives, for the full duration of their lives. Understanding usually only comes when one is confronted with an undeniable refutation of what one thinks they know. And that always takes time. In most cases, on most subjects, it never happens and we go to our graves never knowing the full extent of our lack of wisdom. Usually the best we can do is sort of grasp around the edges of our towering ignorance and try to get a feel for where the contours are, and the better we can do that, the better we can at least guess about what we don't understand, and I think that's the closest to wisdom we can hope for.