I found that this works too.

Map definitions are one for the outer order, and one for a line.

public void DefineMaps(IUmbracoMapper mapper)
{
    mapper.Define<OrderReadOnly, PostOrderDto>((source, context) => new PostOrderDto(), Map);
    mapper.Define<OrderLineReadOnly, LineItem>((source, context) => new LineItem(), MapLine);
}

Then the Map method itself for the order contains a mapping for the line items.

private void Map(OrderReadOnly source, PostOrderDto target, MapperContext context)
{
...
target.LineItems = context.MapEnumerable<OrderLineReadOnly, LineItem>(source.OrderLines);
}

Slightly different to the previous answer as it uses the context value that is passed in.

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