Returns the immediate children of this tag that match a particular type (such as <tr>).
Does not return "grand-children" -- i.e., calling table.Children("tr") will work, but
calling table.Children("td") will typically return nothing because the 'td' tags
are nested inside 'tr' tags.
Don't cache the results of this call.
The tags, or an empty array if none.
To get all rows in a table:
HtmlTagTester[] rows = table.Children("tr");