We develop a public goods game (PGG) to measure cooperation and conditional cooperation in young children. Our design addresses several obstacles in adapting simultaneous and sequential PGGs to children who are not yet able to read or write, do not possess advanced abilities to calculate payoffs, and only have a very limited attention span. It features the combination of haptic offline explanation, fully standardized audiovisual instructions, computerized choices based on touchscreens, and a suitable incentive scheme. Applying our experimental protocol to 129 German first-graders, we find that already 6-year-olds cooperate conditionally and that the relative frequency of different cooperation types matches the findings for adult subjects. We also find that neither survey items from teachers nor from parents predict unconditional or conditional cooperation behavior; this underlines the value of incentivized experimental protocols for measuring cooperation in children.