Learn French: Garder les enfants à l'écart pendant plusieurs heures. - Keep the children away for several hours.

This is an impersonal command. Using the infinitive is standard in French for general instructions, recipes, manuals, and warning labels. It gives a neutral instruction to anyone reading it, whereas gardez addresses a specific person or group directly.

How does the phrase à l'écart work here?

The fixed expression à l'écart means "away", "aside", or "at a distance". It comes from the noun un écart, which refers to a gap, distance, or deviation. Therefore, garder à l'écart is the standard way to say "keep away" or "keep at a distance".

Why do we use pendant instead of pour to say "for several hours"?

In French, pendant is the most common way to express the actual duration of an action, translating to "for" or "during". English speakers often overuse pour, but in French, pour is mostly reserved for a planned or scheduled future duration (like leaving on vacation for three weeks). For a general ongoing instruction like this, pendant is the correct choice.

Could we say plusieurs des heures instead of just plusieurs heures?

No, plusieurs is an adjective here meaning "several" and it attaches directly to the noun without an article or preposition, very similar to how it works in English. You only use plusieurs de when it acts as a pronoun followed by a specific group, like plusieurs de mes amis (several of my friends).