I distinctly remember travelling to singapore when I was about 4 (24 odd years ago now) and sitting in a window seat watching clouds. I must say I have yeat to see anyone put a kid into a car seat to travel on a plane...it must be an american thing. As for needing a car seat while traveling on the MRT in Singapore - I have seen young kids of 4/5 sitting quite happily on the chairs without any discomfort.

As for taxis - I am pretty sure the hotel you stay at will be able t oorder you taxis who have child seats available.