New answers tagged imperativo
If you want to be more polite you can also say: Guarda silencio por favor. which would translate to Keep quiet please.
If you go for singular: Calla. Cállate. as you suggested. If you go for plural: Callaos. but I would not consider that informal. I'm unsure if culturally is considered less rude than in English to tell somebody to shut up. Also I think that many people use wrongly the imperative in Spanish (I mean, native Spanish Speakers), using the infinitive ...
Calla and cállate, as well as (estate) callado will work, meaning exactly shut up and be quiet. Also silencio, and the shush shhh. And maybe chitón. Any of these will work for adult the same as for children. I don't know of any words specifically for children.
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