OPOL seems the best in your case + lots of exposure to Mandarin (amongst family members cousins are best as closer to their age and interests; amongst friends, it's good to "try" and make friends with children who speak the second language by going to clubs and playgroups).
Just a few myth busting facts. In the general population, girls develop language earlier than boys. They're also more "driven" towards language based subjects at school.
Also your first child is more likely to achieve a higher degree of bilingualism than subsequent siblings (so if you have a girl first, chances are higher than for a boy second child for example). Of course these are just general trends. A first baby boy might become highly competent in both languages.
You wonder when is best to start: in the womb! Here's a recent piece of research suggesting that a first language is imprinted on the brain, even if the language is "lost" later in life.
www.theguardian.com/education/2014/nov/20/lost-first-languages-leave-permanent-mark-on-the-brain-new-study-reveals
Another important point. What is YOUR definition of bilingualism? That your children understand? That they can understand and speak? That they can understand, speak, read? Or everything: understand, speak, read, write? And to what level? All situations including formal business type conversation, reading literature or reading transport signs?
Reading and writing as well as learning formal registers will require formal training? Depending how it is presented it can this formal training can act as a support for understanding and speaking or become a hinderance because the child finds speaking easy but starts finding writing and the exercises going with it difficult and tedious with no direct relevance to them.
This brings me to the last point. Keeping an overt relaxed attitude to learning the language is crucial (ie it is simply what we do here in our family, it is our default environment). Unfortunately if language becomes like trying to feed your child greens and met with a constant refusal/battle, it wI'll create resentment and chances of your child being bilingual will rapidly diminish. Try to keep it fun (and never mention you're doing something different/special). We're listening to a song. It happens to be in Mandarin but you don't need to spell that out to your child. You're just listening to a song together. An same for everything: books, tv, taking to the shop assistant, talking on the phone... Nothing special, just what we do.
Good luck!