in DDD, when I don't want navigation property from child to parent, is it still needed at least a property for the ID of the parent?

Question

I have this:

Class Order
{
    long ID;
    ...
}

Class Item
{
    long Id;
    IdProduct;
    decimal Amount;
    ...
}

Perhaps I misunderstood what means bidirectional relation in DDD, because I have read that if it is possible, it is better to have only navigation from parent to child, and not from child to parent, but I don't know if navigation means not to have in the child a property of type order but I need a long property for the ID of the parent or not navigation from child to parent means to have a long property for the ID parent too.

Thanks.