$("table:first > tr:first")
or
$("table:first").find("tr:first")
or
$("table:first").children("tr:first")
or
$("table").eq(0).children("tr").eq(0)
So if I understand the followup question…
$("table:eq(1) tr:has(table:eq(2))")
translates to: get any tr’s in the 2nd table’s if the tr has a 3rd table
or
$("table").eq(1).children("tr:has(table:eq(2))")