Since I don't understand any Mandarin myself, to help him with his homework, first I have to teach him Math and Science in English and then I will teach him the terms in Chinese while learning them myself at the same time. We use electronic dictionaries, apps and the internet to help us.
I find that Math is not so much of a problem for us. Once you have learned some simple math terms in the early primary school years, they continue to be more or less the same in upper primary. The math questions and difficulty gets harder and harder but the terms are more or less the same.
For Math, all you need to do is to master a few basic terms like addition, subtraction, division, multiplication and later on decimals and percentages and money and telling the time. At the most, your child will need to practice some subjective type of questions but they are mostly in ordinary language so once you know the basics you are fine.
However, with Science, if your child does not have a good grasp of the Chinese language and even if they do have a good knowledge of Mandarin Chinese, they will still have to learn and memorize difficult scientific terms in Chinese. It is most challenging because each topic brings with it new terms and words to learn unlike the Math terms which mostly stays the same.
For example, I was going through the Science topic on Respiration with my son recently. With the help of a great video on respiration which we found on the internet, I was able to explain and go through the topic with him very quickly. However, in the next step we had to learn the terms for respiration, respiratory rate, trachea, bronchial tube, diaphragm, oxygen, carbon dioxide etc all in Chinese. This was double our effort and triple our time.
I didn't have such a problem with my girl because she was naturally good in languages so she did most of her studying herself. So, if you have a child in Chinese school, on hindsight, my advise is to make sure they are really good in the Chinese language during the lower primary years so that by the time they have to study more difficult terms and words in the upper primary years, they will face less of a struggle learning.