Both of them are doing Oxford Reading Tree workshops, planning to change into Grammar & Creative Writing for the elder one. At the end, he is Primary 5 now. But for kids in junior primary, ORT would be better and less boring.
You may also check at Kidz n us. Details are available at http://www.kidz-n-us.com. Actually same building.
Another one is ABC Pathways, also in the same building, but fees are much higher. Details are at http://www.abcpathways.com.
My niece has been doing the same ABC on 10/F., of Nan Fung Centre. Details: http://www.abc123.com.hk. Not bad and fees are cheap, but classes may be too full.