For a quick change do:
"MaxRequestsPerChild 4096" to something like: 700 will help. The longer an apache process lives the more resident memory it's going to consume due to mod_php and the like. Also, enable keepalive and place aggresive timeout settings for it:
This will allow each client page request to use one apache process to handle all it's requests where it would otherwise use multiple apache processes. This will cut down on the amount of apache processes running at any given time.
For optimal memory and requests per sec:
Move away from mod_php and use fastcgi, or another app server, instead. Apache processes consume a negligible amount of memory when php pages are served by fastcgi. Not to mention fastcgi can keep persisten connections to your dbase server amongst other things.