¿Vosotros os cepilláis los dientes todos los días?
Step-by-step explanation:
Remember that reflexive verbs need a reflexive pronoun to use (me, te, se, nos, os). It depends on who the verb falls to, a different reflexive pronoun will be used. As the verb "cepillarse" is reflexive, they must be added after the verb when the form of the verb is impersonal (infinitive, gerund and participle).
In this case, the pronoun is "vosotros/as" (you) so the reflexive pronoun is "os".
MissSpanish