Young speakers are not represented adequately in current speech recognizers. In this paper we focus on the problem to adapt the acoustic frontend of a speech recognizer which has been trained on adults' speech to achieve a better performance on speech from children. We introduce and evaluate a method to perform non-linear VTLN by an unconstrained data-driven optimization of the filterbank. A second approach normalizes the speaking rate of the young speakers with the PSOLA algorithm. Significant reductions in word error rate have been achieved.