Adding something like this to your theme.skin file should do the trick:
<portal:mojoTreeView runat="server" SkinID="ChildSiteMap"
or if your theme.skin already has this tag just update the properties on the existing tag.
However that would affect all child page site maps, it is not page specific. We do have a setting in page settings for "Expand on Site Map", however this currently only affects the main site map page. I'm looking at making it possible to use the same setting on the child page site map in the future.
Hope that helps,