Is it permissible to bring two sisters, approximately five years of age, to the men’s mosque, with their heads uncovered and wearing trousers, and pray with them in the row, knowing that they do not play around or distract the worshippers?

Do not ask a five-year-old girl to cover her head, because it is not considered 'awrah (private parts that must be covered). It is permissible to bring boys to mosques if they do not misbehave, and they should be placed at the end of the row, not in the middle.