I do not understand why you want to start with xsd if you do not already have it. If you are free with respect to the scheme, it will start with java code and generate XSD.
:
@XmlElements({
@XmlElement(name = "child1", type = Child1.class),
@XmlElement(name = "child2", type = Child2.class),
@XmlElement(name = "child3", type = Child3.class)})
private final List<IChild> children = new ArrayList<IChild>();
IChild - superClass. XSD-, .