Storing db value into php array with while loop?

The second argument to array_merge should be an array. But to quote the documentation:

mysqli_fetch_array [...] returns an array of strings that corresponds to the fetched row or NULL if there are no more rows in resultset.

In your code, $row is an array. $row[0] is a string.

while ($row = mysqli_fetch_array($result, SQLSRV_FETCH_ASSOC))
{
    $childhbc = array_merge($childhbc, $row);
    echo $childhbc[0];
}

Keep it simple, man

$childhbc = array();
$result = mysqli_query($conn, "SELECT * FROM AdvInfo WHERE Unit = Child");     
while ($row = mysqli_fetch_row($result))
{
    $childhbc[] = $row[0];
}

The less intricate words you use - the better your program works