Evolution is the integrative framework of the life sciences. Even though the topic is often not formally introduced before high school, young children already have various ideas about evolutionary principles (variation, inheritance, and selection) and their underlying key concepts (e.g., differential fitness, reproduction, and speciation). Describing and refining those ideas has increasingly received attention over the last two decades. However, we see two scopes of improvement in the field: (1) There is a need to examine children's ideas about evolutionary concepts holistically rather than focusing at specific aspects. (2) Although research has shown that older students have different ideas about animal and plant evolution, there is little data on children's ideas about plant evolution to compare with their ideas about animal evolution. All of this results in an incomplete record of children's pre-existing ideas that would help to design assessments or interventions. Consequently, we developed a set of questions, about the evolutionary principles and interviewed 24 kindergarten children. Most children had basic ideas about individual variation in animals and plants but experienced a lack of knowledge about the origin of variation. Most children seemed to acknowledge plants as living beings and reasoned equally about animals and plants for most concepts. However, many children failed to reason about reproduction and inheritance in plants because they believed plants would not reproduce sexually. Confronted with a selection scenario, most children struggled applying concepts previously shown on an individual level to a population level. Considering our findings, we propose ideas about how to measure and foster children's pre-existing ideas about evolution.