Minecraft crashes on world load with "java.lang.IllegalStateException: failed to create a child event loop"

Allow me to first offer you a way to find a solution faster. Take the first line of the crash dump and google search it. Then dig.

I found this:

This issue is most commonly caused by a broken installation of Avast! free edition Anti-virus software. A reinstallation of Avast! and a reboot usually fixes it. McAfee anti-virus software (and derivatives) are also know to cause this issue, and currently the only known solution is to uninstall this software to correct the problem.

As a solution according to Mojang bug tracker here

This may be caused by other virus scanners. Try to offer your virus scanner a way to ignore / safelist minecraft. Otherwise disable virus scanner.