From what i know the maths paper is upon 50 marks, why dont they just minus two marks off from 50- make it upon 48.Whatever marks they get, supposing 36 or something.Put 36/48 then re-convert it back to upon 50 marks.
This Primary school kids dont know how lucky they are, at some Secondary schools they will just do the method i mentioned above.It happened to me before.
OR
Give them another 1 MCQ question to do

Conclusion:I just think that giving two marks is not fair, so what if the question is flawed?Do they still understand the concept of solving the question?All people care about is the answer without understanding