The instructor must ensure that everyone can feel safe in the group. This is already set in the ground rules of the group. The instructor must bear responsibility for the group’s good atmosphere and intervene immediately if someone’s behavior or actions violate the positive and encouraging atmosphere.
You should also think of physical safety. Think about possible dangers in the space and make sure you know what to do in an emergency. See that there is a 1st aid kit available and make sure you know the exact location in case of an emergency. Remember that a band aid can often be a great help with little kids!