This paper describes the design, implementation and testing of "Danse-doigts", an edutainment therapeutic application for hemiplegic children. The objective of this program is twofold. Firstly, to allow them to train their fine motor skills on tablet. Secondly, to study the effect of this training on their numerical performance (counting, calculation...). The target population and the objective of evaluating numerical skills influenced the design. The software was developed using standard web technologies but is based on a new parallel programming library written in JavaScript. Applications and libraries are free of charge and easy to install on most tablets.