The point where a message should become a phone call.

Some matters are not email matters and recognising them early saves everything.

There are reliable signals. The subject is likely to cause distress on first reading. There is a real chance of misinterpretation and no chance to correct it in the moment. The matter is complex enough that the message would run beyond what anyone will read. You are on the third exchange without progress. You find yourself rereading a draft repeatedly, unable to get the tone right. Any one of these means the medium is wrong, and no amount of careful drafting will fix a medium problem.

Calling first and writing afterwards is a strong pattern for hard news. The conversation carries the tone, allows questions and lets you hear how it landed. A short written note afterwards records what was agreed, corrects anything that was misheard, and gives the family something to refer back to. That sequence, voice then text, is almost always better than text alone, and it takes less total time than the correspondence it replaces.

One category is not a judgement call. If a matter concerns a child's safety or wellbeing in any serious sense, it is not correspondence at all. It goes to your institution's process, through the people designated to handle it, immediately and by whatever route that process specifies. Do not work it out over email, do not sit on it while you consider wording, and do not let the drafting of a message become a substitute for reporting it.