Back in earlier centuries when lots of America was farmland or forests and modern plumbing wasn't available most people usually bathed once a week. They might have used metal or wooden wash tubs for their baths. The water would have been carried from wells, streams, rivers or lakes and heated over wood burning stoves.
Usually the father would take the first bath, and he'd probably be sweaty and dirty from his labor.
Then the mother would use the same water for her bath, followed by the children from the oldest to the youngest.
By the time the last child bathed, the water must have been very dirty, but carrying and heating separate tubfulls would have been a lot of work.
In California we're having a drought and have been asked to cut back on our water usage. The drought would have to be mighty severe for families to bathe only once a week and share one tub of water. Thank Heaven we aren't having one that serious!