"Take your seat, please!"
Translation:Nehmen Sie bitte Platz!
Firstly, because it's ungrammatical (Platz is masculine, so when it's the object of nehmen Sie!, you would need the masculine accusative form Ihren in front of it, not masculine nominative or neuter accusative Ihr).
Secondly, because it's unidiomatic -- we simply don't say it like that in German. (Sort of the opposite of why "Please take seat!" or "Please take place!" is wrong in English.)
But maybe if I'm telling my child to take its place?...would this translation work?
No; you would say Nimm bitte Platz.
And you'd actually be more likely to say Setz dich bitte (hin) -- Platz nehmen is a bit formal and you probably wouldn't use it to a child.
The equivalent of "Setzt euch bitte" (several persons informal) is "Nehmt bitte Platz".
The equivalent of "Nimm bitte Platz" (one person informal) is "Setz dich bitte".
And then there are the formal versions "Nehmen Sie bitte Platz" and "Setzen Sie sich bitte.
In all those pairs the variant using "Platz nehmen" is a little more formal.
If the children are younger than about 16, you'd say Nehmt euren Stuhl, bitte. with the plural informal form. You wouldn't use Sie to children unless they are nearly adults.
If they're 16 or 17, you would say Nehmen Sie Ihren Stuhl, bitte. -- with nehmen and Sie: the command form for Sie requires the pronoun. Also, Ihren has to be capitalised when it's the polite "your".
Just unusual. But in the way that "Raise the hand, please!" is grammatically correct but unusual in English instead of "Raise your hand, please!".
If you're specifically asking someone to return to their own, previously-assigned seat, you could use einnehmen, though:
Nimm bitte deinen Platz ein! (or Nimm deinen Platz ein, bitte!)
(Those are not currently accepted in this Pearson sentence, as far as I can see.)
grammatically there is no problem with "nimm deinen Platz". But it is not a good translation of "take your seat", which is much better translated as "setz dich/setzen Sie sich" or "nimm Platz / nehmen Sie Platz" (without any article or possessive pronoun!).
If I heard "nimm deinen Platz" I would understand that I am expected to grab my seat and take it with me.