Thanks for the replies, that's really helpful! If I were without the kids I think we'd 100% only stay 2 nights max, but we'll need to take it slow. The flip side to that is that we could extend the stay in Bali for a night and potentially stay somewhere nicer in Singapore for fewer nights.
Basically want to eat and experience the big city vibes. Things like the botanical gardens and the cloud forest dome sound good for the kids. Sentosa island is a bit cheesy, but the cable car is cool and great for the kids. Not 100% sure about the zoo, I guess it depends on how long we end up staying but I know it's amazing.