You are not finding anything as normal aggregation is the SUM of children, factoring in AggregationWeight, AccountType and/or PercentConsolidation of the children as applicable.
To support having parents compute the average of the children, they would also need to provide options as to how to do so, such as including/excluding zeros, weighted/unweighted, mean, etc.
If you write your average formula as a DynamicCalc, you can do so directly in the parent member, or do so in a new base level member that resides elsewhere.
You could also do it as a cube stored calculation written to a base level member, but aggregating pre-computed averages up dimensions can make for some rather non-sensical results.