HttpContext. Текущий получил доступ в статических классах

Вы можете использовать проверку после хука.

public function Postchild(Request $request)
{

   //Define your rules
   $rules =  [
           'child_name' => 'required',
           'gender' => 'required',
           'dob' => 'required',
           'current_class' => 'required',
           'b_group' => 'required',
           'm_tongue' => 'required', 
         ];

  //Validate your data       
  $data = $request->except('_token');
  $validator = \Validator::make($data,$rules);


  $validator->after(function ($validator) {

    //Check the mode of request (Create or Update)
    if(!empty($data['id'])){
        $child_details = Children::findOrFail($data['id']);
        if($child_details->image == null){
          $validator->errors()->add('image', 'Image field is required');
        }
    }
  });
  if ($validator->fails()) {
  return redirect()->back()
  ->withErrors($validator)
  ->withInput();
  }
}