I'm looking to buy a gift for my 8-year-old cousin. From what stores in Manila can I buy cool clothes for kids (around his age) without spending TOO much?

You can try the ground floor of Landmark Makati! They have large bins of kid's clothes that are cute and cheap. SM's surplus shop is super affordable, too!

I agree with Bianca! Super cute and cheap stuff at Landmark. If you're feeling super adventurous, you can try the 888 or 999 mall in Divisoria!