Extract a param from the url

Thursday 01 April 2004 6:07:41 am

hi, i need some help to extract some param from the url page, for example i have this code:
{section}
{*some code here*}
{section-else}
<a href={concat( "/mainpage?", $Child.node_id)|ezurl}>
<IMG SRC={"folder_closed.png"|ezimage} WIDTH="25" HEIGHT="25">
</a>
{/section}

i want to pass a parameter to the main page, but i don't know how since i'm usin the <href> tag, please help me to get the $Child.node_id parameter in the mainpage ....

Any help would be greatly apreciated.

Hugo