Please refer to this link. In short, the saturday English classes (for children aged 4-5) run from 9-12. Each lesson costs $280 and there are ten lessons per term. It is an international school and I think the standard should be very OK. 2 people can enjoy 15% discount. The first term will start on 8 October.
Actually, ESF also have English classes for but I think NIS is more convenient for Tai Po family.