We went in January this year with our kids 12/9/7. Universal Studios is great, but don't torture yourself by dragging them around the whole park trying to get it all done in a day. A 1 day pass for 1x Adult and 2 x Kids with Express pass (you want the express pass, trust me) $437 is more than half the cost of a season pass with express $604, so if you're going for more than 1 day, buy the season pass, works out far cheaper.
We spread the visit to universal across 4 days doing a few areas at a time, by then the kids were exhausted from the heat, so we left to somewhere cooler and then came back the following day. When we were there it was open to 9pm each night... far nicer in the night time, cooler and less busy so if it's open then plan around that if that suits you. We stayed on sentosa so wasn't too hard to do this each day, the MRT and crossing into sentosa if you are staying off sentosa, might make that a little harder and more expensive...
Lots of other activities at Sentosa as well, many indoor which is good during middle of day heat. The Kids role playing place, i forget the name, was well enjoyed, ZIP Line too is good fun!
The children's garden in the botanic gardens is good, MRT stops right out the front of it and it's a nice walk - go early, or late to beat the heat - All 3 kids loved this with all the board walks and animals around as well.
The Gardens by the Bay is great, the nighttime light show is awesome. All three enjoyed that too. There is a water splash park near by as well, bring the togs!
We Didn't go to the Zoo or Aquarium so can't comment on those.
Depending on how the kids are with crowds and how well they pay attention to you (mine are ??? haha) little india was good and the food fantastic!
Our kids are pretty seasoned travelers, we spend a lot of time in tropical places such as Cocos and Christmas Island, however our kids really struggled in the heat in Singapore (to be fair so did I) so just bear in that in mind if your kids suffer from the heat and plan accordingly.
Enjoy!
