tongtong,
If you plan for a long day, sure have to go in the morning lor. If you plan for a short day, go either early morning or later afternoon. Mid afternoon is a killer cos the kid (and adults) will get so tired walking under the hot sun (though it's shaded at most places, but it's just too hot). Or if you plan to let Shaun play a the waterplay area, then morning is better. It's more important to plan which area (what animals) you want to visit most, go there first, cos you (or Shaun) might get tired after 1-2 hours and want to call it a day. Go to the zoo website and check for animal token feeding, definitely worth a try (I personally think better than watching the show), I love the giraffe!