Inclusion Criteria:  Participants who are 2 to 10 years of age inclusive at screening Participants who have a confirmed genetic diagnosis of 5q-autosomal recessive SMA Symptomatic SMA disease, as per investigator's clinical judgement Participants who are ambulant, where ambulant is defined as able to walk/run 10 meters in </= 30 seconds at screening Participants who have received previous SMA disease-modifying therapies may be included provided that: Onasemnogene abeparvovec was received at least 90 days prior to screening. Participants should be tapered off steroids prior to receiving risdiplam. In addition, participants should have normal levels of liver function tests, coagulatory parameters, platelets, and troponin-I at 90 days after administration of onasemnogene abeparvovec or at least 1 month after tapering off corticosteroids, whichever comes later; Nusinersen last dose was received at least 90 days prior to screening; Risdiplam is switched to the investigational medicinal product (IMP) provided by the site  Exclusion Criteria:  Concomitant or previous participation in any investigational drug or device study within 90 days prior to screening or 5 half-lives of the drug whichever is longer, with the exception of those who have completed a risdiplam study, or participated in a nusinersen or onasemnogene abeparvovec study Receiving or have received previous administration of anti-myostatin therapies For Part 1 participants aged 5 to 10 years only: contraindications for MRI scans including difficulties maintaining a prolonged supine position, or any other clinical history or examination finding that would pose a potential hazard in combination with MRI Any history of cell therapy Hospitalization for a pulmonary event within the last 2 months or planned hospitalization at the time of screening Past surgery for scoliosis or hip fixation in the 6 months preceding screening or planned within the next 9 months (Part 1) or 21 months (Part 2) Unstable gastrointestinal, renal, hepatic, endocrine, or cardiovascular system diseases considered to be clinically significant Clinically significant ECG abnormalities at screening from average of triplicate measurement, abnormal findings at echocardiography, or cardiovascular disease indicating a safety risk for participants at the time of screening Any major illness within 1 month before screening Received any multidrug and toxin extrusion (MATE1/2K) substrates within 2 weeks before screening Used any of the following medications within 90 days prior to screening: riluzole, valproic acid, hydroxyurea, sodium phenylbutyrate, butyrate derivatives, creatine, carnitine, growth hormone, anabolic steroids, probenecid, acetyl cholinesterase inhibitors, agents that could potentially increase or decrease muscle strength, and agents with known or presumed histone deacetylase (HDAC) inhibitory effect Clinically significant abnormalities in laboratory test results at the time of screening Ascertained or presumptive hypersensitivity to RO7204239 or risdiplam, or to the constituents of its formulations Clinically relevant history of anaphylactic reaction requiring inotropic support Any abnormal skin conditions, pigmentation or lesions in the area intended for SC injection (abdomen) and that would prevent visualization of potential injection site reactions to RO7204239 Immobilization, surgical procedures, fracture, or trauma to the upper or lower limbs within 90 days prior to screening