Possible ways to say that:
The tiger has scared you?
That tiger started you? It started me too! Ha ha ha.
I know you got scared of the tiger roar, I got scared too, but now I know that he is just a big cat.
That roar of the tiger was fearsome, wasn´t it? But indeed he was just crying for attention!
I don´t mean prompting the child to be fearless to the point of inviting physical harm, though. Parents have to watch their children and see their limits, both of the parents themselves and of their children, of course.
The first sentence could be said in general, the others could be stated to a three-year old, for example, to reassure him/her.
And one has to explain further that in a zoo the tiger is fed, so he roar for attention, a wild tiger in the wood is another thing altogether. So one would have to know how to act there, would hone their (survival) skills before going to the wild.