What causes toddlers to become "musty" without sweating a lot? and what can i do to prevent them from geting musty?

A good bath: Given the activity level of most children, they do tend to sweat - even if not to a noticeable degree visually. A good bath once a day or (even twice in some cases - like after sports) can make a difference. A musty smell can also come from clothing that is not dried entirely after laundering. This mildewy odor can be difficult to get rid of.