Inserting Child Lists into SQL with Dapper

I have the following 2 tables:

public class TestA
{
    public int Id { get; set; }
    public string Name { get; set; }

    public IEnumerable<TestB> Test { get; set; }
}

public class TestB
{
    public int Id { get; set; }
    public int TestAId { get; set; }
}

The idea is I can access all the TestB records related to TestA from my TestA object. What I am trying to figure out is how do I insert into both Test A and TestB, especially if TestB has multiple rows related to Test A. Here is what I tried:

var data = new TestA
{
    Name = "ABC",
    Id = 1,
    Test = new List<TestB>
    {
        new TestB() { Id = 1, TestAId = 1},
        new TestB() {Id = 2, TestAId = 1},
        new TestB() {Id = 3, TestAId = 1}
    }
};

var query = "INSERT INTO TestA(Id, Name) VALUES(@Id, @Name) INSERT INTO TestB(Id, TestAId) VALUES(@Test.TestB.Id, @Test.TestB.TestAId)";

using (var con = conn)
{
    con.Execute(query, data);
}

I know that we can give dapper a list and it is smart enough to enumerate that list and insert individual records, but how do I accomplish the same thing if that list is child of an object? I tried making my values also be @Test.Id, @Test.TestAId but it had exact same error.

The member of type Test.TestB cannot be used as a parameter value