"Why don't you wash your faces?"
Translation:¿Por qué no se lavan ustedes la cara?
I think it's because in English we say "faces" because we're talking to at least two people, so we see at least two faces.
In Spanish we say "cara" because each person has only one face
Is there a reason for the location of ustedes in this sentence? Does anywhere else work?
The "se" comes before the "lavan". The rule for object pronouns (direct objects, indirect objects, reflexives) is that they have to go immediately before a conjugated verb.
If your sentence contains an additional verb form, like an infinitive or a present participle, then you have the choice to put the object pronoun before the conjugated verb or attached to the end of the infinitive or the present participle.
Se lavan la cara. (only a conjugated verb = only before)
Se van a lavar la cara. OR Van a lavarse la cara. (conjugated + infinitive = choice)
Se están lavando la cara. OR Están lavándose la cara. (Conjugated + present participle = choice)
Reflexive verbs usually use definite articles with parts of the body and clothing.
He washes his car. Él lava su carro.
He washes his face. Él se lava la cara.
See my response to lunzie about "caras."
Why is 'Por qué no os lavaís tu cara' not acceptable, ie using the informal second person plural, for children for example?
You incorrectly mixed second person plural with a second person singular possessive. Anyway, your sentence should read "¿Por qué no os laváis la cara?"