Learn French: Ne te salis pas dans le jardin. - Don't get dirty in the garden.

In French, negative commands put the reflexive pronoun before the verb, just like in a normal sentence. So we use te for 'you' (Ne te salis pas). If it were a positive command ('Get dirty!'), it would switch to toi and go after the verb with a hyphen (Salis-toi).

Why do we need the pronoun te at all?

The verb salir means 'to dirty' or 'to make something dirty.' To say 'to get dirty' (literally, 'to dirty yourself'), French uses the reflexive verb se salir. Without the te, Ne salis pas... would mean 'Don't dirty...' and would need an object, like 'Don't dirty the floor.'

I thought we drop the 's' at the end of tu commands. Why does salis keep its 's'?

That rule only applies to -er verbs (for example, manger becomes mange). Because salir is a regular -ir verb, its normal tu form is tu salis. In the command form, -ir and -re verbs keep their final 's'.

How would I say this to a group of children instead of just one?