In order to analyze the sensitivity of children to RF fields and mobile phones in particular, the SAR (Specific Absorption Ratio) defined as the power absorbed by a unit of mass of tissues (W/kg) should be computed based on a numerical model of the head. We propose to build realistic models from 3D-MRI of children heads. The method is composed of two steps. The first one consists in segmenting the main tissues in these images (skin, fat, muscles, cortical and marrow bones, cerebrospinal fluid, grey and white matter, blood, etc). The segmentation is based on mathematical morphology methods which are well adapted to this aim and provide a robust and automatic method requiring minimum user intervention. Using simplified segmented images, the second step concerns the tetrahedral mesh generation. Our method uses almost regular meshes and topological tools to preserve the topological arrangement of the head tissues. A method to guarantee a good geometrical quality is also provided.