A growing standardisation of eating behaviours worldwide is observed, especially in children, notably characterised by the increasing consumption of both unsustainable animal- and plant-based ultra-processed foods (UPFs). The objective of this study was to identify five high-impact eating behaviours (HIBs) in a French child population (living in Clermont-Ferrand city and surroundings, n = 92, aged 6–15), that is, behaviours to change for the strongest positive impact on health and sustainability. For this, both multidisciplinary top-down and participatory bottom-up approaches has been carried out. First, from European and French dietary guidelines, desk research, European researchers’ evaluation, and expert interviews and workshops the technical potential (for their impact on health and environment), plasticity and feasibility of 40 HIB were first quantified (top-down approach). Secondly, in the French living lab (LL), from photo-voice and focus groups with children, online surveys, meetings with families and children, interviews with stakeholders, and assessments of the behavioural change potential of the French LL children’s target, five HIB were finally selected (bottom-up approach). Therefore, among the initial 342 European recommended eating behaviours, the selected HIBs for our specific population were: limiting the consumption of UPFs (notably those high in added salt, sugars and/or fats), eating 0–3 servings of meat/week, eating three servings of legumes/week, choosing primarily wholegrain than refined cereals, and choose water instead of sweetened beverages. These five HIBs constitutes a solid basis to perform next LL participatory actions, notably to test HIB feasibility/plasticity, and implement them in real life conditions through transversal actions with non-researcher.