Hola Brandon,

I think you are referring to the use of the imperative form of estar where the endings change; in the example that’s you have quoted we see “estén” rather than “están.” In fact under the sub heading “Ser atento vs Estar atento” in the lesson you will see the example below, where the indicative spelling “estáis” has become “estéis.”

Niños, necesito que estéis atentos. Esta lección es muy importante.

Children, I need you to pay attention. This lesson is very important. 

You are correct about this being a command, and as a command / instruction and as such it takes the imperative mood, which uses the subjunctive forms in some of the conjugations. I have pasted a link to the general lesson on this below, but it would be good to look up a good on-line dictionary like WordReference where you can see both forms at the same time, across all conjugations.

Forming the imperative for usted/ustedes/nosotros/nosotras (affirmative and negative commands)

I hope I have understood your question and that this helps.

Saludos.

John

Indeed, "usted" imperatives of '-ar' verbs always [?] adopt the a>e vowel-change [such that they look like a subjunctive], so "estar" is simply following that rule.

It might be worth mentioning that the "estéis" in "necesito que estéis atentos" is a [present] subjunctive - because it is governed by 'necesitar' [a "verb of influence"].