Bad smell is usually caused by infection or bacteria or toxins. The first thing I would do would be to offer more water especially early in the day. Are there enough wet nappies?

Are their rooms too hot? 18'C is actually a comfortable temperature to sleep in even for a baby.

Could it be something in their food/formula?

Maybe try a steamy shower instead of a bath? Showers are a cleaner way of removing oils/sweat etc from the body. I tend to shower with my children, especially babies. My DH carries them out afterwards to dress. Families do have "family smells" but it shouldn't be unpleasant.

I think offering more water during the day will be of most help. Even young babies can have cooled boiled water.