To have children you have to get married first.
How to Create Kingdom and Have Children.How to Learn Blocking, Attacking (Guide).Smith and Forge (Crafting, Getting, Refining).Which Faction to Choose (Empires, Vladian, Sturgia, etc.).Map Guide (Silk Road, Trading and Goods).Main Questline Guide (Search, Banner, Empire).
This is a guide on how to create the kingdom and have children in Mount & Blade II: Bannerlord.