class  Parent {
    String parentField;
    List childList;
}

 class  Child {
    String childField;
}

                    <display:table name="parentList"
                                   pagesize="100"
                                   id="parent"
                                   cellspacing="1"
                                   requestURI=""
                                   defaultsort="0"
                                   sort="list"
                                   width="100%">

                        <display:column align="left" property="parentField" />

                        <display:column>

                            <c:set var="childList" value="${parent.childList}" scope="request"/>

                            <display:table name="${childList}" >
                                <display:column property="chieldField"/>
                            </display:table>

                         </display:column>


                    </display:table>