Learning te reo and making it the main language within the home isn't something that happens overnight and it can't be solved by going to a few wānanga or classes, although these will certainly help. Changing the main language of a home is a hard thing to achieve and it can be very frustrating especially during times of stress to stick to the commitment when a lot of times it will seem a lot easier and faster saying something in English.
This is the hard part. Persevere. It's times like this when the kids are playing up, when dinner is smoking on the stove, when someone's been hurt, when you are running late, when anything which increases the stress levels in your house will really test you and your family's commitment.
Try and pre-empt these stressful situations and take some time out and work out what sort of phrases you might need just in case some of these things happen and keep them in your back pocket for just such an occasion. It will definitely bring down the stress levels if you already have the reo sussed out for what you need to say.