"Children are not supposed to be here." The children either are or they are not, it works to make contractions like so; Are not - Aren't: We are not (aren't) going to do that. Can not - Can't: We can not (can't) do that. Do not - Don't: Do not (don't) do that! Should not - Shouldn't: I should not (shouldn't) have done that. To throw some examples out for you!

Hello Dear, you should say ''Children are not supposed to be here.'' The other sentence is incorrect. :) Have a nice day :)