The call for researchers to obtain children's informed assent, prior to their participation in medical procedures and research, has increased over recent years and parallels moves to implement child-centred approaches to health care. This article describes the processes used to include children in developing a research information sheet and assent form for use in future research into children's understandings of their surgery and hospital experiences. The process involved primary school children aged between six and 12 years. Children worked in small groups to consider information to include in these documents. Their words were collated to construct the research information sheet and assent form. Working with children resulted in documents that were more understandable for their intended audience. The article includes discussion of 'language', 'understandability' and 'readability'; concepts that researchers seeking to work with children need to come to terms with if they are to obtain 'informed assent'.