You could use a non-blocking waitpid and simply time all the children, then signal/kill them if they go over whatever limits you set.

(from perldoc -f waitpid)

use POSIX ":sys_wait_h";
 ...
do {
   $kid = waitpid(-1, WNOHANG);
} until $kid > 0;
[download]

status of the child is in $?

HTH!