Just a little matter...
Everyone has his/her/their own problems.
Everybody has his/her/their own problems.
As far as I know both
everyone and
everybody are singular so... I'm not convinced that I can use
their in such a case.
On the other hand, I don't think it's correct to use
his or
her for boys&girls together.
However, one of them should be right

. I don't have any idea, so please, someone help me.
And here appears
someone 
. Like...
Someone has/(have?) left her/his/their door opened.
Woah... that's a hard nut to crack for me!