"The kids ride the bus to school every day."
Definitely 们 is not necessary. Chinese has no plurality and the pronouns are the only terms that 们 matters.
们 would be used only when we want to emphasize the meaning of a collective. If I speak about the children in my family I would not use 孩子们, but if I speak about the children in a town then I would consider using it, to represent an age class in the society.
这个孩子 refers to "this child" and 这些孩子 refers to "these children". Both are natural usage and indicate clearly whether it is single or plural.