Factors that might account for the probability of children being exposed to secondhand smoke compared to those who are unexposed and characteristics associated with the urinary cotinine level (UCL) of those who are exposed were investigated. All households in a German region with a child aged 3 years or younger (n = 3,570) were invited to participate in a study that tested the efficacy of an intervention for reducing secondhand smoke exposure. In 1,282 households, at least one parent reported daily smoking. Among these, 915 (71.3%) participated in the study. For data analyses, we used a two-part model. Characteristics of the households associated with SHSE of the youngest child were analyzed, as well as characteristics associated with UCL among those exposed. Exposure to secondhand smoke was defined using a UCL ≥ 10 ng/ml. Secondhand smoke exposure was detected in 57.1% of the samples. Nursery attendance was associated with secondhand smoke exposure, in addition to the number of smokers living in the household, extent of home smoking ban and parental education. Among children exposed, nursery attendance, season of urine collection and age of the child were associated with UCL. Consideration of seasonal smoking behavior and a child's age at the time of intervention may increase attention to the adverse health effects of secondhand smoke exposure.