I hope you have a great stay in Singapore, but I thought I'd highlight a few concerns:

1) Zoo day - I'm sure your child will love it, but It does seem quite inefficient to travel to and from the Zoo twice on 1 day. Cab fares are about US$10-15 from orchard road for each journey - so you are looking at US$40-60 on transportation! Perhaps better to have lunch (albeit without the animals) and follow through with the night safari.

2) Sentosa - underwater world, riding on the luge hanging out at the beach, the possibilities are endless visit - http://www.sentosa.com.sg/sentosadiy/ and prioritize... but after the day out, junior might be too exhausted to put up with shopping; Should you decide to push on, then perhaps doing it all at Suntec city might offer you some respite from the heat. You can get on the duck tours which is amphibious so you get a boat ride and cruise all in one... then you can shop where you get off! Or alternatively squeeze in a ride on the Singapore flyer (giant ferris wheel) at sunset.

Saturday & Sunday are very busy at places like the Zoo & Sentosa (just so you know).

For you son, you might like to consider the Singapore Science Centre out at Jurong East (close to the Chinese/Japanese Gardens, Omni Theater and Snow City)

Thanks for your advise. Spore does seem a lot of fun. Looking forward to my trip.