There are a few steps in ensuring that a child will be able to get the correct answers for MCQ science.
The child needs to have a strong grasp on the concepts. This is somehow non-negotiable. Without having a grasp on the concepts, the child will not be able to draw any information from his/her memory bank.
Read the question carefully/slowly while taking note of the terms being used. These words/terms will click with the concept to the various topics. Once the link has been made to the concept, information retrieval could then be carried out.
Go through the 4 options that has been given as answers. A common approach by students would be to look for the correct answer. However, most of the time, the options given are very similar and they are very close. What I would suggest would be to apply elimination strategy. Eliminate the options that is definitely wrong. By doing so, you will narrow down the number of options before arriving to the answer. Most students do not practice this but instead, they immediately try to find the correct answer.
These three steps look very basic but it is something that many students do not actively practice, especially step number three. Hopefully, this helps.