Pivot...UnPivot??? How to fix this bad data setup

Use this. That line was not needed.

let
    Source =Table.FromRows(Excel.CurrentWorkbook(){[Name="Table2"]}[Content]),
    Custom1 = [a = List.Skip(Table.ToColumns(Source)), b = {a{0}} & {a{3}} & {a{1}} & {List.Skip(a{1}) & {null}}, c = Table.FromColumns(b, {"Parent ID",	"Event","Start Date", "End Date"}) ][c]
in
    Custom1

I think what yoy are asking for is result for a group of Parent ID. Then use this

let
    Source =Table.FromRows(Excel.CurrentWorkbook(){[Name="Table2"]}[Content]),
    Custom1 = Table.Combine(Table.Group(Source, {"ParentId"}, {"All", each [a = List.Skip(Table.ToColumns(_)), b = {a{0}} & {a{3}} & {a{1}} & {List.Skip(a{1}) & {null}}, c = Table.FromColumns(b, {"Parent ID",	"Event","Start Date", "End Date"}) ][c]})[All])
in
    Custom1