Lack of assent from a child
Imagine you are doing a research project with a 7 year old child, which requires some blood to be removed from the child. The parents agree for the child to take part, and the child also agrees. When the nurse comes towards him with the needle to take the blood, he screams in fright and says he doesn't want to take part. Do you need to withdraw the child from the study?
This is not an easy question to answer as it will depend on a number of factors: can the child provide their own consent; how persistent is their fear of taking part, could they wait with the option to participate later if their fear subsides.