I do think it's mean to intentionally tease a zoo animal like that. Yes, I'm sure no harm came of it and I'm not saying that the parents should be asked to leave the zoo. I do think it would have been right to ask them to move on. Most zoos have rules that you aren't allowed to agitate the animals and this lion is clearly agitated (even if it's just playful-type agitation).

The animals we see are gifts. We should treat them as such.

Additionally, it's probably not great to encourage such a dangerous animal that it's OKAY to behave in such a matter towards humans. It's a danger to the keepers, veterinarians and other staff that work behind the scenes every day. Those individuals know how to exercise caution and every measure is taken to keep even more than a good wall between human and beast...but you still don't want an animal learning it's okay to be aggressive (even playfully).