If the child showed work and obtained the wrong answer, the question is marked wrong.
If the problem has more than one part to the answer, such as part a. and part b., the problem can be marked half wrong, or one-third wrong, as appropriate, depending on how much of the problem the student got right and how much wrong. This is at the discretion of the teacher.
Beginning in the child's second mathematics book, if the child gives the answer but does not label the answer with the correct units, the problem is marked wrong. If the question has units, the answer must have units. A number without a label, or with the wrong label, is wrong. For example, if the question asks "How many pencils..." then the answer must be labeled with "pencils." You may think this is not important, but a multimillion dollar space probe was in danger of crashing into a planet for failure to label the answers. When the space probe was launched a decade earlier, the engineers failed to label their numbers. No one could remember if the space craft was programmed to navigate in "feet" or "meters." Ten years of work and millions of dollars were in danger of being ruined because the numbers were not labeled.
If the teacher cannot read what the child wrote, the child either needs to rewrite it legibly or it is wrong for failure to communicate the correct answer.
If the child neglected to do a problem, the child needs to do the problem or it is marked wrong.
Do these standards seem steep? In the adult world, failure in any of these areas can result in discipline or loss of a job. This is one of the reasons that home school students step into the adult world with confidence - they've been living under adult standards of performance their entire lives, adjusted only for age appropriate ability.
Why not use these standards? It is more work for the teacher. It is more work for the student. If teaching in a school setting where parents are involved, you can expect parents to complain that the teacher is "too strict," "too harsh," "too demanding," etc. Do you want excuses, or do you want students who can do mathematics well?