C# Generic Parent Children Definition

I'm trying to design a parent/children definition in C#, but receive some compilation errors, which are not clear for me:

namespace TestProject1
{
    public abstract class BaseElement
    {
    }

    public abstract class Child<T> : BaseElement
    {
        public BaseElement Parent { get; set; }
    }

    public abstract class Parent<T> : BaseElement
    {
        public IQueryable<BaseElement> Children { get; set; }
    }

    public abstract class ParentChild<TParent, TChild> : BaseElement 
        where TParent : Parent<BaseElement> 
        where TChild : Child<BaseElement>
    {
        public void AddChild(TChild child)
        {
            child.Parent = this;
        }
    }

    public class Root : Parent<Trunk>
    {
    }

    public class Trunk : ParentChild<Root, Branch>
    {
    }

    public class Branch : ParentChild<Trunk, Leaf>
    {
    }

    public class Leaf : Child<Branch>
    {
    }
}

One of the compilation errors is in line public class Trunk:

Error CS0311 The type TestProject1.Root cannot be used as type parameter TParent in the generic type or method ParentChild<TParent, TChild>. There is no implicit reference conversion from TestProject1.Root to TestProject1.Parent<TestProject1.BaseElement>.

But in fact there is a path from Root to Parent<BaseElement> as Root inherits from Parent<Trunk> and Trunk is a BaseElement due to the fact that BaseElements inherits from ParentChild, which is a direct successor from BaseElement.

The other compilation errors are similar, but for the Branch type in Trunk and the Trunk and Leaf parameters of Branch.