We have neighbors in our building who unfortunately have a seriously disabled child. While I empathize with the family and how hard it must be for them, the child litters the building considerably. Since this is communal space that we all pay to keep clean, I would expect the family to regularly clean up after the child, which I imagine they do in their own home, rather than waiting for the weekly cleaning of the building. Should I convey this to the family, and if so, how?
This is a very sensitive topic. First, let's pray for the well-being of the child and for the emotional and financial well-being of the family, which I'm sure they need in order to care for the child.
As you suggest, they're most likely aware of the problem, as they surely have to clean up after the child in their own home.
So if they nevertheless leave the litter in the building it must be that it's very difficult for them, for whatever reason, to regularly clean up after her there. This might be out of embarrassment, lack of time or energy, or perhaps even subconsciously to make note of their plight.
One might suggest that they pay for a second cleaning during the week, but given the great expense they must already have in caring for the child's needs, this could be too much of a burden on them, as well as being embarrassing – unless the other tenants agreed to pay for another cleaning themselves, attributing the need to the children of the building in general.
However, I recommend not saying anything to the family about it at all. I understand that the litter must bother you, particularly since it's in an area others pay to clean. But their situation is just too sensitive and difficult to judge without knowing personally what it's like, G-d forbid.
Rather, I suggest that you transform the discomfort you feel over the litter into ways of serving G-d. For one, it should be a regular reminder for you to thank G-d for having healthy children, being challenged by this child alone out of your home, rather than it being your own child in your own home. In addition, the regular anguish you feel over the situation should be channeled into a regular opportunity to empathize with the family and pray for the welfare of the child and for that of the family and their ability to care for him or her.
Another thing to consider is that we too "litter" our environs with all types of improper thoughts, speech and actions; yet we neglect cleaning up after ourselves and expect G-d to overlook the messes we make. Therefore, we should also overlook the shortcomings we find in others and rather use such opportunities to correct and improve ourselves instead.