Sirius actually thought that was a decent idea; it might help reinforce the idea that they really couldn't be left to their own devices.
"Could say it was their part of the decorating. I mean, we don't want them feeling left out, yeah?" He grinned wickedly, watching as James levitated the reindeer up to the top of the tree.
Really though, he thought they'd clean it up and head to lunch; it seemed like a good plan to him.