MacDonalds is kind of fun if you are 5.
SAFRA at Toa Payoh is good - you can climb on indoor climbing frames and then book a private room for food etc.
You can book Monkey Cafe in Orchard Towers
We booked Jonathan, the balloon man, Kids Entertainment or something which was well enjoyed
Rainforest cafe was good when it was open.
One of the private children's education places will lease out space for a party. Kidz Place? in Liang Court.
KFC in the Zoo with a private space was certainly unusual and fun for the children
Someone else's function room....
thats about it for ideas