There need not be a either Mandarin or English debate. Many European are bilingual. Chinese can be the same. Considering the number of business and projects China has invested overseas, English looks like the medium of choice to run its projects and manage it's overseas business.
The most expensive English class is not going to help unless the child has someone to continually communicate with in English. It gotta be fun. And that someone is probably their age. Use Weixin 威信, QQ etc and allow them to communicate and record their English conversation and learn from it. Language should be fun else they won't learn it.