As it stands one parent can take a child as a passenger on a motor bike without the others consent. This form of travel per mile is 38 times more deadly than by car. There is a risk of serious harm which should not fall down to only one parents consent.
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A child may not understand the importance of holding on etc on what is already a high risk travel method. It takes a 30 second google to see I am not the only parent to feel the same. As a parent all we want is for our child to be safe and feel safe.