Learn Spanish (Latin America): El niño mira hacia atrás.

hacia means "towards" and shows the direction of a movement or a gaze. The phrase hacia atrás literally means "towards the back" or "backwards". While English just says "looks back", Spanish often includes hacia to make the direction clear.

Could we use hasta instead of hacia in this sentence?

Not with the same meaning. hasta means "as far as" or "until" and sets a limit or destination. If you said hasta atrás, it would mean looking all the way to a specific endpoint, like the very back row of a room. hacia atrás just points out the general direction he is looking.

What is the difference between atrás and detrás? Can I say hacia detrás?

You should stick to atrás here. atrás is used for direction or movement (back or backwards), which fits perfectly with the action of looking. detrás is used for a fixed location to mean "behind" something, usually followed by de (like detrás del árbol, meaning "behind the tree").

Is hacia a form of the verb hacer (to make/do)?