National law in Germany requires that, whenever possible, children must provide their assent before participating in clinical research. However, there is still academic debate about many fundamental components of assent in order to address, for example, the age or stage of development respectively, at which children should be asked for assent. Furthermore, only a few studies approach the child’s assent to research participation empirically. We present empirical findings from a population-based survey in Germany on parents whose children were first diagnosed with childhood cancer in 2005. The survey’s primary objective was to evaluate what the child’s assent to research participation means to parents who gave consent on behalf of their minor child. In particular, we wanted to better understand what parents think about the requirement of seeking assent, how to assess the children’s competence to give assent and who should be in charge of it. Our empirical findings indicate that parents want to give children a voice in the decision-making regarding research participation. Even though the child’s competence to rationally understand the research protocol is primarily discussed in the literature as the most important precondition for a valid assent, the surveyed parents emphasise the child’s maturity instead. Given that maturity is regarded as a gradual process, parents want to have a say in assessing it. From this, it follows that parents develop and use a decision-making model that establishes appropriate roles, individual choices and responsibilities for the children, the parents and the physicians.