Stephanie, there are several things you can do to acknowledge infant mortality. If you have some info, such as a name, a gender, a date, a location, the name of the mother, or an actual birth or death record, you can create a profile for the infant. I personally think it's appropriate to do that for an infant who lived for at least a short time, if you have some or all of that data, (although I have seen comments in G2G by some who disagree). When you don't have a name, you can use something like "Unnamed Infant" or "Unnamed Daughter" as a given name. For a stillbirth, or a case where you don't have enough info to justify a separate profile, you can add a comment in the Bio of the mother's profile.
You may know also that in the U.S. Census records for 1900 and 1910, married women were asked how many children they were the mother of and how many of those children were living. I have encountered a number of cases where I can find no record of a child that was claimed, and I suspect that a stillbirth or infant death may be the explanation. In that case I also add a comment to the mother's profile noting the inconsistency.