As Alan said on his first comment, using the specific pronoun "os" that refers to "vosotros" is also correct, but, as explained in the lesson, this structure "a + infinitive" is used in a more general way. When the verb is a pronominal one, e.g lavarse, and still using this structure in a general way, directed to people in general, you keep the general reflexive pronoun (se).
Here are other similar examples:
¡A ducharse! Have a shower!
¡A peinarse! Comb your hair!
¡A concentrarse! Concentrate!
I hope this clarifies your doubt.
Thanks for your comments.
With formal pronouns usted or ustedes we don't use this structure, we would use the subjunctive form instead.
"-os'' vs '-se'
I think I understand this, but to clarify, using 'se' in general has nothing to do with formal/informal? For example, in a quiz on this issue, the command was addressed to "niños..." and the answer required both versions as answers (os, se). I thought, since it was addressed to kids (for example, a parent saying something), that the only correct answer would be "...os". So, using "se" is okay with any group?
Yes, you are right. The command with "se" at the end doesn't imply any formality.
The difference between using -se or -os would be that the last one would be a bit more specific when you have a few people in front of you, these people being kids or adults, it doesn't matter, but both are equally correct.
Kwiziq answer "A lavarse" is correct in a general sense, the the quiz question addressed to command to "Vosotros dos" i.e two specific people so it is still confusing.
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