SpecFlow and complex objects

I would say that Marcus is pretty much correct here, however I would write my scenario so that I could use some of the extensions methods for in the TechTalk.SpecFlow.Assist namespace. See here.

Given I have the following Children:
| Id | Name | Length |
| 1  | John | 26     |
| 2  | Kate | 21     |
Given I have the following MyObject:
| Field     | Value      |
| Id        | 1          |
| StartDate | 01/01/2011 |
| EndDate   | 01/01/2011 |
| Children  | 1,2        |

For the code behind the steps you could use something like this will a bit more error handling in it.

    [Given(@"I have the following Children:")]
    public void GivenIHaveTheFollowingChildren(Table table)
    {
        ScenarioContext.Current.Set(table.CreateSet<ChildObject>());
    }


    [Given(@"I have entered the following MyObject:")]
    public void GivenIHaveEnteredTheFollowingMyObject(Table table)
    {
        var obj = table.CreateInstance<MyObject>();
        var children = ScenarioContext.Current.Get<IEnumerable<ChildObject>>();
        obj.Children = new List<ChildObject>();

        foreach (var row in table.Rows)
        {
            if(row["Field"].Equals("Children"))
            {
                foreach (var childId in row["Value"].Split(new char[]{','}, StringSplitOptions.RemoveEmptyEntries))
                {
                    obj.Children.Add(children
                        .Where(child => child.Id.Equals(Convert.ToInt32(childId)))
                        .First());
                }
            }
        }
    }

Hope this (or some of this) help to you

For the example you have shown I would say you're cuking it wrong. This example looks more suitable to write with nunit and probably using an object mother. Tests written with specflow or similar tool should be customer facing and use the same language as your customer would use to describe the feature.