Instead of following after her, Cas waited until he noted that she'd committed to the slide and doubled back, waiting until she'd reached the bottom. It was a simple distance to close and he wouldn't have needed his Grace or his wings to make it to her. Once she reached the bottom, he tapped her lightly on the shoulder. He was careful to not touch her more than he needed to, respecting her "personal" "space." It was still not a concept he fully understood, but he was playing by the rules he'd agreed to.
Leaning down some, he half-frowned, but the very faint smile on his face was present. For those who knew how to look for it. "Tag," he rasped.
There was a moment where he very nearly teleported away out of habit, but then remembered himself. Instead, he reached back and took hold of the ladder behind him and climbed up onto the play equipment himself, waiting to see what Helena did next. He had several options, but only a few of them would be correct. He also considered letting her win, but he felt she might see that as an offense. She was not a child - that was very important to both of them.
So instead, he crossed the play equipment and jumped down from where the monkey bars began.