We went to Safari World in Dec, and my daughter is just 2 months old. We spent around 3 hours there, not going to all the shows. I think it's really worth going, especially when we are so close to the animals when driving the car.
I don't think 2 hours will be enough, esp if you are going in your own car, as you can stop whenever you see an animal... There's still another part that's worth going as well, you can see polar bears, kangaroos, dolphin, etc. Kids will like it~~ (at least mine loved it very much)
We hire a private van, as we have 4 adults + 1 kid. The price is about HK$500 for the transfer (you can go to any place from 9.00 am to 5.00 pm) include a tour guide, and the Safari tickets is about HK$120 which include lunch.
It’s more expensive to take private van, but I don’t quite trust the taxi in Thailand.