public static class TreeNodeExtension
{
	public static IEnumerable<TreeNode> FindNodes(this TreeNodeCollection nodesCollection, Func<TreeNode, bool> predicate)
	{
		var nodes = nodesCollection.Cast<TreeNode>();
		return nodes.SelectMany(n => FindNodes(n.Nodes, predicate).Union(nodes.Where(n2 => predicate(n2))));
	}
}

return 	from n in nodes
		from child in n.Nodes
		where predicate(child)
		select child;

var checkedFilesNode = treeView1.Nodes.FindNodes(n => n is FileNode && n.Checked == true);