you're doing it first thing in the morning or at a venue with a pool I'd say forget it. Too hot! Loads of hot sweaty red-faced children and adults.
or it'd get rained off!
You could try doing it at Pasir Ris park - it's quite well shaded and fairly quiet if you do a weekday morning, there's a big children's playground (although I'm not sure if there's much for small children - you'd have to go and check it out first to see what you think) and there are a few shelters there that you could set up your food etc in.
The other place I can think of is the children's garden at the Botanics - I'm not sure if they take bookings as such, but you might be able to go there with a picnic and let the kids have a play