$_SESSION dosent’t work

So when i use variable $ _SESSION['wp_childs'] = $ _POST['childs'] in my plugin, nothing changes in my storage session.

i use session_start(); at the top of that all pages; for use $ _SESSION but i can’t store my new session variable.

So i want juste create a new variable session and use it after.

if you have any ideas for store all my variable without global variable i take it.

Thank you !!!