Entity Framework not saving modified children

Frustrating, this. Here's a pair of related objects, as generated by database-first Entity Framework:

public partial class DevelopmentType
{
    public DevelopmentType()
    {
        this.DefaultCharges = new HashSet<DefaultCharge>();
    }

    public System.Guid RowId { get; set; }
    public string Type { get; set; }

    public virtual ICollection<DefaultCharge> DefaultCharges { get; set; }
}

public partial class DefaultCharge
{
    public System.Guid RowId { get; set; }
    public decimal ChargeableRate { get; set; }
    public Nullable<System.Guid> DevelopmentType_RowId { get; set; }

    public virtual DevelopmentType DevelopmentType { get; set; }
}

Here's the code that I'm calling to save a DevelopmentType - it involves automapper since we differentiate entity objects from DTOs:

    public void SaveDevelopmentType(DevelopmentType_dto dt)
    {
        Entities.DevelopmentType mappedDevType = Mapper.Map<DevelopmentType_dto, Entities.DevelopmentType>(dt);
        _Context.Entry(mappedDevType).State = System.Data.EntityState.Modified;

        _Context.DevelopmentTypes.Attach(mappedDevType);
        _Context.SaveChanges();
    }

In my user interface, the most common operation will be for a user to look at a list of DevelopmentTypes and update their DefaultCharge. So when I test this using the above code, it runs without error, but nothing actually changes.

If I pause in the debugger it's clear that the changed DefaultCharge is being passed into the function, and that it's attached to the DevelopmentType to be saved.

Stepping through it, if I change the value manually inside visual studio, it does save the updated value. Which is just even more confusing.

Monitoring the database with SQL Server Profiler reveals that update commands are issued only for the parent object and not for any attached objects.

I have other similar code elsewhere that functions as expected. What am I doing wrong here?

EDIT:

I have discovered that if you do this prior to the call to SaveDevelopmentType:

        using (TransactionScope scope = new TransactionScope())
        {
            dt.Type = "Test1";
            dt.DefaultCharges.First().ChargeableRate = 99;
            _CILRepository.SaveDevelopmentType(dt);
            scope.Complete();
        }

The change to Type saves, but the change to ChargeableRate does not. I don't think it helps, massively, but thought I'd add it.