How to call a child component method from parent in React Native typescript

hello i'm new in React Native with typescript and in this example code i want to call testChild function from child Component in parentCall function in parent class and i dont know how to keep reference of each child component i cant change child component code how can i call testChild for each child component ?

class parent extends React.Component {
   public render(): JSX.Element {
        return (
         <child/>
         <child/>
               )
 public parentCall(): void{
   //call testChild for each Child component ??
}

}
class child extends React.Component {
 public render(): JSX.Element {
        return (
               <Text>Text</Text>
               )

 public testChild (): string{
   return Data ;
}