We've done the day trip a couple of times and the children have loved it (first time ages 4 and 6). I would recommend the guided tour to get the most out of the trip. The tours are done by a group of very knowledgable volunteers. Participants are split into small groups when the ferry arrives and then each group heads off onto different trails. Unless you're really into NZ birds and know what to look for, the guides add an awful lot to the trip. The tours finish at the info centre and then you have plenty of time for lunch and to head off to another area of the island before the ferry returns.
The 'cafe' amounts to an area where you can make a cup of tea or coffee and perhaps buy a chocolate bar from the shop, but there is a large sheltered area where you can eat your own supplies. Snacks and drinks can be bought on the ferry.
We've only booked the ferry a day or two ahead for our trips out there.