How do you break circular associations between entities?

my first time on the site so apologies if it's tagged incorrectly or been answered elsewhere...

I keep running into particular situation on my current project and I was wondering how you guys would deal with it. The pattern is: a parent with a collection of children, and the parent has one or more references to particular items in the child collection, normally the 'default' child.

A more concrete example:

public class SystemMenu 
{
    public IList<MenuItem> Items { get; private set; }
    public MenuItem DefaultItem { get; set; }
}

public class MenuItem
{
    public SystemMenu Parent { get; set; }
    public string Name { get; set; }
}

To me this seems like a good clean way of modelling the relationship, but causes problems immediately thanks to the circular association, I can't enforce the relationship in the DB because of the circular foreign keys, and LINQ to SQL blows up due to the cyclic association. Even if I could bodge my way round this, it's clearly not a great idea.

My only idea currently is to have an 'IsDefault' flag on MenuItem:

public class SystemMenu 
{
    public IList<MenuItem> Items { get; private set; }
    public MenuItem DefaultItem 
    {
        get 
        {
            return Items.Single(x => x.IsDefault);
        }
        set
        {
            DefaultItem.IsDefault = false;
            value.DefaultItem = true;
        }
    }
}

public class MenuItem
{
    public SystemMenu Parent { get; set; }
    public string Name { get; set; }
    public bool IsDefault { get; set; }
}

Has anyone dealt with something similar and could offer some advice?

Cheers!

Edit: Thanks for the responses so far, perhaps the 'Menu' example wasn't brilliant though, I was trying to think of something representative so I didn't have to go into the specifics of our not-so-self-explanatory domain model! Perhaps a better example would b