Is it thank yous? I want a character to say...

'Now, don't forget your thank yous' (she is talking to a group of kids)

Cheers.

Yes, I'd say Thank Yous

It's all one phrase there... Don't put an apostrophe, whatever you do.

My inner pedant had a nice moment of wondering whether the plural would be "Thanks You", as in Queens Regnant and Attorneys General... But the grammar isn't the same.

Ooh, thanks, Emma - i'll put the capitals in, as well.

If used in dialogue your characters can say anything you want them to say. Anything, so long as it's consistent.
'Yous lot, listen-up! I willnae stand fir this constant improper use of grammar in ma class - can ye no get it intae yir thick skulls, that the only "you" sounding word that's got an 's' for a plural is the one used fir female sheep,'said Mr McTavish.
Michael

If used in dialogue your characters can say anything you want them to say.

Yes, that's a very important point. People do speak ungrammatically, and say lots of things which are not technically correct English, so dialogue can reflect that. It has to be consistent with the character, of course, but within that constraint, anything goes.

Alex

People do speak ungrammatically, and say lots of things which are not technically correct English

I ain't never done nothing like that, mush. Not never.

i watched Dad's Army recently and was thrilled to hear Jones stick 'innit' on the end of a sentence, in the way that is popular now, eg , 'we'll thrash those Jerries, innit?'

Clive Owen started it all!

Clive Owen started it all!

Or Clive Dunn, even.

(By the way, did you know that David Jason was originally in the frame for that role? He was ruled out because at the time he was very young, even though Clive Dunn wasn't anywhere near as old as he was made up to appear.)

Of course, Alex! Good old Clive Dunn.

I didn't know that! Interesting.

"Thanks You" as in sisters-in-law? No I agree, quite different. Thank yous.