<parent>
    <child>
        <random>123</random>
    </child>
    <child>
        <random>234</random>
    </child>
</parent>

import com.fasterxml.jackson.core.JsonProcessingException;
import com.fasterxml.jackson.databind.ObjectWriter;
import com.fasterxml.jackson.databind.node.ObjectNode;
import com.fasterxml.jackson.dataformat.xml.XmlMapper;

import java.util.List;
import java.util.Random;

import static java.util.Arrays.asList;

class MyPojo {
    public int random = new Random().nextInt();

    public static void main(String[] args) throws JsonProcessingException {
        List<MyPojo> list = asList(new MyPojo(), new MyPojo());
        XmlMapper mapper = new XmlMapper();
        final ObjectWriter writer = mapper.writer().withRootName("parent");
        ObjectNode node = mapper.createObjectNode();
        list.forEach(x -> node.putPOJO("child", x)); // <= does not work, replaces values instead of adding them
        String s = writer.writeValueAsString(node);
        System.out.println(s);
    }

}