Where is the best place to buy children clothes in SG? Any recommended outlet?

Depends on what you are looking for. Are you brand conscious? I've bought cheap home wears for my kids from pasar malams and in Chinatown :-) Uniqlo and H and M has a lot of really nice and cheap options too. Try Uniqlo and go to their "damaged" rack section. You will be able to find slightly damaged item in rock bottom prices :-)