public class A : IOrderable { }

public class B : IOrderable { }

public class Term
{
  public ObservableCollection<A> aCollection { get; set; }
}

public class Year
{
  public ObservableCollection<B> bCollection { get; set; }
}

public abstract class InfoListViewModelBase : ViewModelBase
{
  public ObservableCollection<IOrderable> childModels { get; set; }
}

public class TermViewModel : InfoListViewModelBase
{
  public TermViewModel(Term t)
  {
    this.childModels = t.aCollection;
  }
}

public class YearViewModel : InfoListViewModelBase
{
  public TermViewModel(Year y)
  {
    this.childModels = y.bCollection;
  }
}