You can create a stored procedure for this. The user that creates the procedure should have ALTER SYSTEM privileges granted directly to them. Then you grant EXECUTE privileges on your procedure to the users you allow to kill sessions. There are a few considerations when building this procedure, like what if they have multiple sessions, etc.
I would also scan thru some of the sample scripts that can be found in tech books and online sites - maybe there's a canned procedure written already for this.