This is a Phase 2, multicenter, open-label, dose-escalation study to determine the safety, tolerability, and identify potentially effective dose(s) of TYRA-300, a fibroblast growth factor receptor (FGFR)-3 selective tyrosine kinase inhibitor, in children 3 to 10 years of age with achondroplasia with open growth plates that will examine three cohorts of children: the Sentinel Safety Cohort, Cohort 1, and Cohort 2.