The reference to CHILD is a legacy compatibility reference that is used in aggregation functions. It will automatically be included in your solution if you defined roll up metrics using the Metrics Formula capability prior to the release of Metric Expressions. The CHILD reference dictates that Shibumi should aggregate from one level down in the solution hierarchy. E.g., sumIf(CHILD, Total_Revenue__m).
Note: The CHILD reference is not available when writing new expressions and is only provided to support backward compatibility with Metric Formulas.