Myoclonic jerks in this age group can be due sleep myoclonus, myoclonic epilepsy and several other metabolic/ genetic issues. You can consult a pediatric neurologist or an Epileptologist regarding further evaluation and management. Some of the myoclonus can be benign myoclonic Epilepsy, which gets cleared after a certain age.
Next Steps
Consult a pediatric neurologist/ Epileptologist