With her sister, artist Sarah Peterson, they have been creating props and scenery.
The Duchess is familiar with party planning as her parents run a party decoration business, Party Pieces, in the UK.
The Duchess selected Rainbow Place for a visit because of her ties with children's hospices at home.
Kate and William's diary
• Monday, April 7 - Wellington
Arrive at Wellington Military Terminal. Welcome ceremony at Government House.
• Wednesday, April 9 - Wellington
Plunket nurse and parents' group hosted at Government House.
• Thursday, April 10 - Blenheim/Wellington
Visit Seymour Square War Memorial in Blenheim and Omaka Aviation Heritage Centre. State reception at Government House.
• Friday, April 11 - Auckland
Visit RNZAF Base, Whenuapai and Viaduct Basin. Match-race sailing of America's Cup yachts on Waitemata Harbour.
• Saturday, April 12 - Waikato
Duke visits Pacific Aerospace, Duchess visits Rainbow Place children's hospice. Visit Cambridge.
• Sunday, April 14 - Dunedin/Queenstown
Service at Cathedral Church of St Paul, The Octagon. Attend youth rugby tournament at Forsyth Barr Stadium. Arrive Queenstown, visit Amisfield Winery. Shotover Jet trip.
• Monday, April 14 - Christchurch
Visit CTV memorial park and 2015 Cricket World Cup event. Visit Wall of Remembrance at Wigram Air Force Museum.
• Wednesday, April 16 - Wellington
Visit Police College and Civic Square. RAAF flight to Sydney.