Practice for modeling class - multiple container relationship

Question

I have a class that could:

1. Have multiple types of containers
2. Have multiple types of implementations

and what I did to model so far is:

public interface ChildClass {
    Container getContainer();
    ...
}

and one of its impl (have a total of two, could be extended though):

public class ChildImplA implements ChildClass {
    ...
}

I have (as you might have already guessed) a Container interface:

public interface Container<T extends ChildClass> {
    List<T> getChildren();
}

and two interfaces extending Container:

public interface ContainerA<T extends ChildClass> extends Container<A> {
    List<T> getChildren();
}

and

public interface ContainerB<T extends ChildClass> extends Container<    
    List<T> getChildren();
}

The generic T refers to the implementations of ChildClass and a sample implementation of any of the last two would be like:

public class ContainerAImpl implements ContainerA<ChildImplA> {
}

I have a couple of related questions regarding how to (properly) model this in the database, and whether the design so far could have been better:

1. I have a child_class table with container_a_id and container_b_id with foreign keys to container_a and container_b tables. Though one of the foreign keys has to be null at all times, is this still a better practice than having container_id that could reference multiple tables?
2. Should I introduce a generic, say P to ChildClass as follows:

public interface ChildClass<C extends Container> {
    C getContainer();
    ...
}

and have 4 implementations of it in total (instead of 2 for the moment) as follows:

public class ContainerAChildImplA implements ChildClass<ContainerA> {
    ...
}

Please bear in mind that implementing 2nd question also brings in further complications (e.g. implementing a few other DAOs for different types of ChildClass implementations, instead of just doing:

getJdbcTemplate().update(child.getContainer() instanceof CintainerA ?
                        "INSERT INTO child_class VALUES container_a_id = ?" :
                        "INSERT INTO child_class VALUES container_b_id = ?" ,
                child.getContainer().getId());