Purpose of the study: The main purpose of this study is to determine the safety and evaluate the effect of a once weekly dose of TransCon CNP in prepubertal children with achondroplasia in China. Study Treatments: TransCon CNP is an investigational (new) drug, which means that it is currently being tested, and therefore is considered experimental. TransCon CNP is designed to provide a sustained exposure of active CNP by subcutaneous (under the skin) injection once weekly. The Randomized Period of this study is a double-blinded and placebo-controlled. "Placebo-controlled" means that some participants will receive injections that don't contain any TransCon CNP (placebo injection - no active ingredient). "Double-blinded" means that neither the participant nor the study doctor will know which treatment the participant will be receiving, except in an emergency. After completion of the Randomized Period the trial participant may be invited to take part of the Open-Label Period of this study. "Open-label" means that all participants will receive injections that contain TransCon CNP; regardless of which treatment (TransCon CNP or placebo) was assigned during the 52 weeks (1 year) Randomized Period/blinded treatment period. It also means that both the participant and the study doctor will know which treatment, and which dose the participant receives.