[jQuery] why is :first-child selecting sub elements?

[Liming]

Hello all, I have a lay out like so

<div id="childtest_toprow">
                <div id="subtest_1">
                        <div id="subtest_1_1"></div>
                        <div id="subtest_1_2"></div>
                </div>
                </div>
                <div id="subtest_2">
                </div>
</div>

I want to select the first child of "childtest_toprow" which is
"subtest_1"

when i do

$("div[id ^= 'childtest_' ] :nth-child(1)").each(function() {
                                        alert( $(this).attr("id") )
});

I get "subtest_1" AND "subest_1_1"

what am I doing wrong?

Thanks

ps: i'm using firefox 3 and IE 7