You can say that, it's correct English. The question is asking you to use a comparative with 'polite.' Much politer' or 'more polite' are the possible comparatives. Your children are politer than your brother's children. In your answer, you have modified the comparative with 'much''. Consequently the interpretation of your answer is that the degree to which your children are more polite than your brother's is larger. There is a difference in meaning between your answer and those given in the book. It is correct, but on this occasion not what the mark scheme was looking for.