@Hosfac:
All heirs must be the firstborn of the firstborn. In your example, the second child is not the heir, so it wouldn't matter if he/she had a child before the first child did or not. And as Salty said, it's not a good idea to let any others marry and have children because nobody can ever move out of the Dynasty house. All must live there until their natural deaths. For example, if the founder has 2 children and both marry and have children, that puts 8 people in your house at the 3rd generation. The founder's spouse would probably die soon after, which would allow your 3rd generation to marry, but no children could be born until another dies. You have to think long term when planning your strategy. Don't bite off more than you can chew at the start.