Have you considered the Wanganui Top Ten holiday park? They have motel units which might be basic but they'll be entirely functional, and I cannot imagine a more child friendly environment as they have large grounds and various activities. It should also be quiet heading into winter, and if I was the owner I would welcome a long term rental over this period, so would probably negotiate a good rate? The park is located a short drive from the centre next to the river in a quiet and seemingly safe suburb. Good luck.