Two datasets inside a same tablix

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SSRS

Using SSRS , need to generate report, here i have two datasets one for main display & second dataset for displaying 2 sub categories of first datasets.

So the output will be, first dataset's each row may or may not contain its respective 2 sub categories[second dataset].

so here i need to display main row with its repective 2 sub catgories if present for that main row.

Any idea of how to display as Main dataset as Parent row with its repective 2 sub categories row next to it if present

two datasets used also is their possible to display with two tablix , one tablix present inside other or so

sample output :

1 , parent row1
Child 1
child 2
parent row2 ----- in case of no child , child 6, child 7 will not be displayed , only Parent row2 alone displayed
Child 6
child 7
parent row3
Child 3
child 4