We use a very easy system, first up one should own a galvanised bucket (has many other uses as well) heat water in this on any fire then you need a pump and shower rose, buy from supercheap, but i can advise on others if you need.
You will need a 12v supply ( we have a 900 amp battery pack, now about 8 years old from super cheap)
When heating the water you can keep an eye on the temperature, especially when bathing the young ones