parent child extension

This might take 2 to 6 hours on 120000 rows.  Start it before leaving work for the day and it should be done when you return.

let
    Source = Excel.CurrentWorkbook(){[Name="Table1"]}[Content],
    tbl = Table.Buffer(Table.RenameColumns(Source,{{"Child", "Child level 1"}})),
    chld = List.Buffer(tbl[Child level 1]),
    nokids = Table.SelectRows(tbl, each not List.Contains(chld, [Parent] )),
    haskids = Table.Buffer(Table.SelectRows(tbl, each List.Contains(chld, [Parent] ))),

    lsta = List.Accumulate({1..12}, {nokids, #table({"Parent","Child level 1"}, {})}, (s,c)=>
            let 
                lastcol = List.Last(Table.ColumnNames(s{0})),
                ct = List.Count(List.RemoveNulls(Table.Column(s{0},lastcol)))
            in   
                if ct>0 then  
                    let  
                        tbl1 =Table.NestedJoin(s{0}, {"Child level " & Text.From(c)}, haskids, {"Parent"}, "Child level " & Text.From(c+1)),
                        tbl1e = Table.ExpandTableColumn(tbl1, "Child level " & Text.From(c+1), {"Child level 1"}, {"Child level " & Text.From(c+1)}),
                        tbl1k = Table.SelectRows(tbl1e, each Record.Field(_, "Child level " & Text.From(c+1)) <> null)
                    in   
                        {tbl1k, s{1} & Table.SelectRows(tbl1e, each Record.Field(_, "Child level " & Text.From(c+1)) = null)}
                else s),
    tbla = lsta{0} & lsta{1}
in
    tbla