Child2 table has nothing to indicate which row should be chosen when say row2 is requested - you cannot rely on the order in which you think the rows sit in the table. This scheme is highly unconventional and as it stands it won't work. Have you considered taking a more conventional approach and normalising your tables?
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Understanding and using APPLY, (I) and (II) Paul White
Hidden RBAR: Triangular Joins / The "Numbers" or "Tally" Table: What it is and how it replaces a loop Jeff Moden