Sirius hadn't even realized what James was doing when he handed him Toby before disappearing just as quickly as he'd shown up. Watch Toby while he took Allison flying? There were plenty of people around Sirius had to wonder how it was that he ended up being the one chosen to watch Toby.
He gave the boy a doubtful look, trying to decide just what it was he was supposed to do with him. "Flying?" Well James had taken him flying, he could probably do that too.
"Yeah alright." He mounted his broom and helped Toby on. "Make sure you hold on," He reminded Toby, though he'd be keeping hold of him as well.