Are the hung children printing anything before they hang? If not, are they blocked trying to output (i.e. check with 'ps'). Normally when a child is forked, you'll want to redirect its output somewhere else (e.g. a file or a pipe to the parent process) to avoid having the system block them when they try to output.

One quick and dirty way to check if it's blocking on output is to temporarily comment out the print statements in the children. If things start working, then this is probably the problem.