system
Given the following post, respond as if you're commenting on Reddit:
human
Why is causing children suffering as punishment via physical harm unacceptable but causing them suffering via mental harm (by say, isolating or boring them) acceptable?

Obligatory “I’m not saying we should do x but just want to know why exactly”.
gpt
How's three choices of how you want to tackle this:

If you want to make a purely philosophical case, you also need to ask what your ethical framework is. Is it consequentialist (achieving desirable ends), deontological (a defined set of rules), normative (what people just.. think?). the level of acceptability can change depending on what stance you take.

but as this seems to be a more concrete and practical question, we can fall back to the concrete principles of bioethics

* nonmaleficence: With boredom/isolation there is usually no intent to harm. With physical punishment, if we go with the idea that corporal punishment leads to no desirable outcomes (as can be argued for the child's development), and it comes from an attempt to violate, then the common sense moral judgement that this is not good or desirable
* autonomy: this is where it gets tricky, but its far easier to say that children have the ability to choose what they want to do when they're isolated or bored. When you beat a child, it is conventionally by force and restraint. Second, someone might say that the body is one's personal property and essential for living. If you beat the shit out of a child, you are damaging that which does not belong to you, and you are comprimising their ability to sustain their lives. 
* beneficence: is the intent to better the child or the situation?
* justice: Could the punishment be just? **That is largely dependent on whether you think the child is morally responsible for their actions**

That last part is the clincher. Many times we presuppose children are helpless and incapable of being accountable for their actions. If they can't be blamed for what they do, is punishment ever acceptable? Your call on that one. Based on that, you can plug and chug to see whether boring a child and beating a child fall in the same moral vein. 

Third, if you approach this from a politico-philosophical perspective (in the broad sense of examining about how groups of people should fall into place and interact and whether/or how they should be "ruled) you could just say it's precedent and that's ok, or that certain forms of isolation are wrong and others aren't, or that this has little political content, or that you cannot delineate when non-physical control of children goes from ok to all-out not okay. 

 there are infinite ways to skin this cat.