You can create a plugin like this:
class plgExtensionsEnableMyPluginsplgExtensionEnableMyPlugins extends JPlugin
{
public function onExtensionAfterInstall($installer, $eid)
{
if ($eid)
{
$db = JFactory::getDbo();
$db->setQuery("UPDATE #__extensions SET `enabled` = 1 WHERE `extension_id` = $eid AND `type` = 'plugin'");
$db->execute();
}
}
}
Install and enable the above plugin before installing your plugins.