The aim of the study was to gain knowledge about what role the foster carers’ biological children play in foster family care and also gain a deeper understanding of the biological children´s significance in a foster family care arrangement. The aim of the study has been conducted through the perspective of the Social services. The data collection consists of eight individual qualitative and semi-structured interviews with social workers who are currently working with foster families there biological children are living at home. The new childhood sociology and the concept of participation serve as the base of the theoretical framework and have been applied to the analysis of this study. The study shows that the respondents have a positive attitude towards the biological children´s presence in the foster families. They value the biological children´s function in the families as important and should thus not be forgotten. At the same time, there are no clear guidelines for how the social worker should work with foster carers’ biological children. The contact between the social worker and the biological children mostly goes through the children’s parents. The role of the biological children is not always clearly defined in the foster family care arrangement and their involvement can be perceived as insignificant. Furthermore the respondents terminates that no special support designed for biological children in foster families exists. Several respondents also reflect upon that they want to avoid burdening the biological children with responsibility that is not theirs to bear. The role of the biological children in the foster family care and their significance in the foster family care arrangement concludes therefore acknowledged by the social worker but not always recognized.