Developmental changes in physiology during childhood influence drug dosing. Failure to account for these changes leads to improper dosing, which is associated with decreased drug efficacy and safety in children. Population physiologically-based pharmacokinetic (PBPK) modeling offers the opportunity to predict optimal drug dosing based on physiologic parameters adjusted for developmental changes. PBPK models are mathematical constructs that incorporate physiologic processes with drug characteristics and genetic variances to characterize the dose-exposure relationship across the age continuum. These models integrate drug-specific (e.g., metabolism, protein binding) and systems-specific (e.g., organ size, blood flow) information to predict the effect of different factors (e.g., age, genetic variants, disease) on drug exposure. By accounting for these factors and using data from clinical trials to confirm the modeling, PBPK models can reduce the number of children needed for clinical trials while maximizing dose-based efficacy and safety. This trial will evaluate a platform to prospectively validate population PBPK models in children. The study drugs, clindamycin and Bactrim (aka TMP-SMX), are ideal candidates to evaluate population PBPK models in children due to their differing physico-chemical properties and elimination pathways. In addition, a trial of clindamycin and TMP-SMX has broad clinical applicability, as both drugs are among the most commonly used agents to treat gram-positive infections in infants and children.