When he's kneeling down like that, Madoka can hop on relatively easily - and so she does, not wanting to make him stay in that position for too long. Especially since he did give her permission, saying that he doesn't mind it, so surely it should be okay, right..?

(.. hopefully? Madoka is so insecure at times that she can't help but worry about it, even after people have given her permission.)

She makes sure to cling on to him tightly, just so he doesn't have to worry about her falling off. But even as she's doing so, she can't help but ask:

"You have.. great grandchildren..?"

Wangji doesn't look that old.