Primary school homework is a common practice internationally, historically viewed as an independent child activity, but more recently recognised as a family accomplishment. Parental involvement in homework has been principally discussed in relation to general and fixed typologies, with parent behaviours categorised into pre‐defined ‘types’. This paper challenges that framing by theorising homework as an interactional event. It illustrates that parental involvement is not simply determined by parents’ involvement ‘type’; rather, as an interactional exercise, homework is negotiated in‐the‐moment by parent and child, in linguistic, embodied and material ways. Based on a corpus of 74 video‐recorded homework sessions collected in England and Italy, and adopting discourse analysis, the article reveals that parents display their understanding of what counts as ‘appropriate involvement’ and, at the same time, locally negotiate this with their children, often adapting their involvement practices to meet children's explicit or implicit requests. We present this phenomenon as a ‘flexible line of involvement’ which can shift during each interaction, according to local negotiations embedded within the homework encounter. This shapes the unfolding event, as the parent's moment‐by‐moment responses to their child may result in them ‘crossing the line’. By demonstrating the locally negotiated fluidity of parental involvement, this article highlights the complexity of parent–child primary homework, moving beyond common assumptions that homework is either a lone child's activity, a task solely shaped by schools, or the result of fixed types of parental involvement.