Definitely Abel Tasman and head over the hill to Golden Bay if you have time - stunning beaches, cafes, walks, nice farm park at Takaka. Drive down through the Buller gorge to Westport. Stop en route and venture over the huge swing bridge and take the flying fox back. Visit the lovely seal colony at Westport - great easy walkway and stunning beach. Drive down to Greymouth (not the best place itself) for Punakaiki and Shanty Town. Drive through Arthur's pass?
It depends how relaxing you want it to be really. You could just spend the week in Golden bay and be totally chilled at the end or drive to see some sights. Alternatively if you want to see some of Abel Tasman, Kaiteriteri is a beautiful base and is the start of lots of boat trips into Abel Tasman - you probably wouldn't be able to do the kayaking into the park because of Kyles age but you can do family kayak hire at Kaiteri itself - nice playground there too.
Kyle would probably enjoy the seals and also the gold panning/ train ride at Shantytown (just going on what S likes as they're the same age).
Contact me on FB if you'd like more info as I haven't been on here for ages. It does get pretty busy around Jan so you may need to book accom early - especially around Golden Bay, Abel Tasman area.