jquery: use (“parent > child”) with variables

For example I have a <div class="first"> containig other <div> 's and selected first one and pushed him into variable

variable = $('div.first')

And now I want something like this

$(variable + " > div")

I know, it seems stupid but what if I had an array containig table rows and I need to access columns inside a specific row:

var tableRows = $('table > tbody > tr');
var tableBodyCols = $(tableRows[i]+'> td'); // Doesn't work :(