function funccell6()
{
 var y=document.getElementById('myTable').rows[2].cells
 var newtext =y[1].firstChild.nodeValue
document.getElementById('cell6').value = newtext
}



<input type="button" value="Put the contents of cell 6 in this text input using the 'child' DOM notation" id="cell6botton" onclick="funccell6()" style="border:transparent">
<input type="text" value="" id="cell6" ><br>