S.G.
M.,
My background in interior decorating has taught me many things. People no matter who they are, are unable to smell their own body odor or household odor. We become immune to our own odors, and that of our family members, when that odor is a daily odor. So, it is very possible that the parents of this child are unable to smell what other people may find repulsive.
There are several possibilities here. Due to the economy it could be that this couple is having trouble. Maybe they are having trouble with their water bill, or having trouble purchasing the proper cleaning agents. That would mean doing less laundry and taking fewer baths.
It may be that this couple is unaware of how important cleanliness is to a baby. It may be that the father is responsible for bathing and that he is slacking off, since the mother seems to be clean. There is also one other thing that could be going on. I have known couples that seem to be operating like most couples to those on the outside. But on a closer look you may find that the mother lacks the motivation to care for the child, although she cares for herself. I have known more than one couple that fell into this category. I have seen in each case, the father step up to the plate and begin to care for the child in the home. They feed, change and care for the baby more than the mother, becoming the primary care taker. However if the father is willing, but does not grasp the entire concept, you end up with this kind of situation.
I am not sure telling any parent that their child smells, is the best way to handle this. There are ways around it though. You could explain that you have a sensitivity to smells, and odors. Many people have varying degrees. Explain that you are not sure if it is the detergent, or the soap, but that you are having trouble with it. This puts the blame on an item of unkown origin, and your sensitivity, not on their parenting skills.
I would ask if you could keep some clothes at your house that you can launder, with a detergent that you know you are not sensitive to. I would not tell them that you are going to bathe her on a regular basis, this would relieve them of their duties to do so. I would however let them know that on occasion if one of the children becomes sweaty, or seems fussy, you may bathe them. This allows the babt to feel more comfortable and sleep better when napping.
I hope this helps some.