Using Whatif Measures and Average on Measures

You can create separate measures for child average and parent average, with both standard and weighted versions, then combine them into an overall version which you can use in your reports and which will use the correct measure depending on what level of the hierarchy it is at.

Child Avg =
IF(
    ISINSCOPE( ParentChild[Child1] ),
    VAR currentChild =
        SELECTEDVALUE( ParentChild[Child1] )
    RETURN
        CALCULATE(
            AVERAGE( ParentChild[Val] ),
            REMOVEFILTERS( ParentChild ),
            TREATAS( { currentChild }, 'ParentChild'[Child1] )
        )
)

Child Weighted Avg =
IF(
    ISINSCOPE( ParentChild[Child1] ),
    VAR currentChild =
        SELECTEDVALUE( ParentChild[Child1] )
    VAR weighting =
        LOOKUPVALUE( WhatIf[Value], WhatIf[Key], currentChild )
    RETURN
        [Child Avg] * weighting
)

Parent Avg =
VAR currentParent =
    SELECTEDVALUE( ParentChild[Parent] )
VAR result =
    AVERAGEX(
        CALCULATETABLE(
            ADDCOLUMNS(
                SUMMARIZE( ParentChild, ParentChild[Parent], ParentChild[Child1] ),
                "@avg", [Child Avg]
            ),
            TREATAS( { currentParent }, ParentChild[Parent] )
        ),
        [@avg]
    )
RETURN
    result

Parent Weighted Avg =
VAR currentParent =
    SELECTEDVALUE( ParentChild[Parent] )
VAR result =
    AVERAGEX(
        CALCULATETABLE(
            ADDCOLUMNS(
                SUMMARIZE( ParentChild, ParentChild[Parent], ParentChild[Child1] ),
                "@avg", [Child Weighted Avg]
            ),
            TREATAS( { currentParent }, ParentChild[Parent] )
        ),
        [@avg]
    )
RETURN
    result

Avg = IF( ISINSCOPE( ParentChild[Child1] ), [Child Avg], ParentChild[Parent Avg] )

Weighted Avg = IF( ISINSCOPE( ParentChild[Child1] ), [Child Weighted Avg], ParentChild[Parent Weighted Avg] )