Take care of the children's break first
On arrival, visit the toilets, farmers' market, and bakery in that order. If the children want to go outside, head to Pukupuku Park; in poor weather, switch to the onsen. Either choice extends your stop without much fuss.
Parking → toilets → farmers' market → bakery → Pukupuku Park or the onsen.
Check the weather and ground conditions for the park, and the onsen's closed days and admission hours.