Do you need to have explicitly the number of the child in the destination schema?
if you have in source :
<Child>
<Name>AA</Name>
<Item>
<Detail>AB</Detail>
<Detail>AC</Detail>
</Item>
</Child>
Do you need in Destination
<Data>
<Child>
<Name>AA</Name>
<Item>
<Detail>AB</Detail>
</Item>
</Child>
<Child>
<Name>AA</Name>
<Item>
<Detail>AC</Detail>
</Item>
</Child>
</Data>
or
<Data>
<Child_1>
<Name>AA</Name>
<Item>
<Detail>AB</Detail>
</Item>
</Child_1>
<Child_2>
<Name>AA</Name>
<Item>
<Detail>AC</Detail>
</Item>
</Child_2>
</Data>
And is <Child_3> is necessary (because there is only 2 details)
If this is first destination, you could use the mapper and use the looping functoid linked between the Item in source and the Child in Destination and link Name and Detail from source to dest.
If this is the second destination, you need to define completely the destination xsd and you can use the count functoid and index functoid to fill the correct destination node.