Introduction One of the ethical issues surrounding birth cohort studies is how to obtain informed assent from children as they grow up. What and how parents tell their children affects children's future choices about the study, yet few studies have focused on parents’ influence on children. Objective This study examines parents’ attitudes towards telling their children about their participation in a specific birth cohort study. Methods We conducted surveys and in‐depth interviews with the parents of children who participated in the “Japan Environment and Children's Study” (JECS), which follows children from the foetal stage to age 13. Results Forty‐four mothers and 23 fathers answered the survey, and 11 mothers and 3 fathers participated in in‐depth interviews. Parents’ attitudes towards “telling” were categorized into 3 communication styles depending on their perception of the risk/benefits for their children. Most parents predicted that the study would benefit their children and preferred “directive telling,” which we divided into “empowered telling” (provides children with a positive identity as participants) and “persuasive telling” (attempts to persuade children even if they express reluctance as they grow). A few parents, weighing the study's potential risk, preferred “non‐directive telling,” which respects children's choices even if that means withdrawing from the study. Discussion While “directive telling” may lead children to have positive associations with the study, children should also be told about the risks. Investigators can provide materials that support parents and give children age‐appropriate information about their participation, as well as ensure opportunities for children to express their feelings.