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By default this is set to undef, and the feature is disabled if the value is undef or 0 (zero).
When enabled (by setting the value to the maximum allowed runtime in seconds), this feature sets up a process timeout (alarm) for each NMIS child process (in the documentation also often called a "thread"), which terminates the process if it remains stuck or running for too long. This serves as a safeguard for nodes that are totally unresponsive or much too slow in responding, and which would otherwise keep the collect operation from completing.
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Please note that the operation of the max_child_runtime feature is independent of ignore_running; that is, you can use neither, one, or both of these features at the same time.