Foster care is an important concept in our society and is preferred over other institutional care. However, including a foster child in the family, brings consequences to the whole family, including the birth children. Fostering is known to come with both positive as negative effects for birth children and parental concerns for the wellbeing of birth children often contribute to foster care breakdown. The aim of this study is to explore foster child characteristics and the acceptance of foster children, by birth children and to compare mothers’ perceptions to birth children’s own perceptions. 139 Belgian birth children and their mothers were questioned with a self-assembled questionnaire. Regarding foster child characteristics, foster mothers and birth children reported that 20–21% of the foster children presented emotional problems, 14.5–20.5% behaviour problems and 25.2–33.1% signs of hyperactivity. Foster mothers seem to overestimate the problems of their foster children compared to the problems of their birth children. Subsequently, even though they both mostly reported average levels of cohesion and adaptation, foster mothers gave higher scores on cohesion in the family and significantly lower scores on the adaptation fostering asks, compared to the scores of their birth children. Regarding acceptation, most birth children indicate that they have a positive attitude towards the foster child and would have difficulties with an ending of the foster care placement. However, both foster mothers as birth children mostly report low levels of acceptation. Two backward linear regressions were done to become a predictive model of the acceptation of the foster child by the birth child. Results indicate that it is important to involve birth children in the recruitment and to support them throughout the whole foster experience. Behaviour problems of foster children should be screened earlier and more effective interventions to lower problem behaviour should be researched and deployed.