The aim of NAPPA is to develop new models to improve our understanding of how children process ('metabolise') penicillins, to help us discover if we can improve the dosing regimens used for children in the future. The study population will involve children and neonates who need a penicillin antibiotic as part of their normal National Health Service (NHS) care, and who also need blood tests or intravenous access. Therefore the antibiotic study can be done without the need for any extra invasive procedures, and there will be no change to the medication therapy received by participants. In addition, NAPPA will evaluate whether it is feasible to measure antibiotic levels on a tiny drop of blood put onto absorbent paper ("dried blood spot"). This allows the sample to be stored and tested later on. NAPPA will compare the level of the antibiotic using this method with the level measured in the usual way, to show if this is a reliable method that could be used routinely in the future.