<?php

use Symfony\Component\Serializer\Normalizer\NormalizerInterface;
use Symfony\Component\Serializer\Normalizer\scalar;

class ChildNormalizer implements NormalizerInterface
{
    public function normalize($object, $format = null, array $context = array())
    {
      return [
        "name" => $object->getName(),
        "age" => $object->getAge(),
        ...
        ];
    }

    public function supportsNormalization($data, $format = null)
    {
        return ($data instanceof Child) && is_object($data);
    }
}