It's not the zoo's fault.
They warned about lifting children for a better view.
Not every zoo employee has a tranquilizer gun. The zoo only uses tranquilizer guns when they know something deadly is going to happen, and in this case they didn't know the parents would do this. Also, tranquilizer darts dont instantly make animals stop what they're doing and sleep, and it requires time to actually aim the thing at the animal, but this all happened in an instant.
Anyways, I bet you that child is in a better place now, and is watching the animals and his/her parents and smiling.