It doesn’t take long for a child to acquire English. You will notice that your Māori speaking child has times when they “play” with English and want to practise their English on you. What is important is that their Maori language ability keeps pace with their cognitive development. Children need to be able to express themselves verbally and they will do it in whatever language they find easiest. If they can convey their thoughts more readily in English, that will be the language they choose to speak. A child who cannot express their ideas and feelings, quickly becomes frustrated and will start misbehaving.
The solution to this is to ensure that, from a very young age, the child has plenty of high quality language exposure and is provided with the language needed to express their most complex thoughts.