In Danish outdoor kindergartens, children are spending most of the day outdoors often in forests or similar nature environments. These children are assumed to be healthier than children attending conventional kindergartens, however, factors related to choosing a specific type of kindergarten may explain the differences. To better understand this, we aimed to investigate parents reasons for choosing either outdoor or conventional kindergartens based on a mixed-method participatory Concept Mapping approach, and further if parental socio-demographics and early child characteristics differed prior to enrolling children to either type of kindergarten using a cohort register-based approach. Parents of children attending outdoor kindergartens (n = 23) weighed reasons such as “physical setting, outdoor life, and freedom of movement” high, whereas “a good first impression of the kindergarten” was an important reason for parents choosing a conventional kindergarten (n = 22). In the register-based approach, 2434 and 2643 children attended outdoor or conventional kindergartens, respectively. The parents choosing outdoor kindergartens as well as their children differed according to most investigated characteristics, including origin (maternal non-Western: 4.2% vs. 21.9%, p < .0001), educational level (maternal long education: 45.6% vs. 33.0%, p < .0001), prematurity (5.1% vs. 7.1%, p = 0.004) and sex (females: 43.5% vs. 48.6%, p = <0.0013). In conclusion, parental reasons for choosing kindergarten as well as parental socio-demographics differed substantially among kindergarten type. These differences might cause selection bias if not considering when comparing health outcomes among children attending different kinds of kindergartens.