Select a div's children id using jquery

Select a div's children id using jquery
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#1

I have code like…

var i = $(this).children().eq(0).attr('id');

I want to know that in the variable i, has i already select its id? or i have to write this code again to select its id like…

var id = $('#i');

I think I am doing something wrong or my coding is wrong format.
I really don’t know what to do to select the id of first child of a certain div.