What is the ruling of the Sharia on his ex-wife preventing him from seeing his daughter after their divorce and her giving birth to their child, given that the daughter is now six years old, despite his repeated attempts to see her?

If the wife prevents the father from seeing his daughter, she is in error, even if she is the custodian, because the Shariah permits the visited child (mahdun) to be visited and seen by the other party. It is advisable to try and resolve the issue through wise intervention from the wife's family, otherwise, by employing every legitimate means to obtain this right. These issues highlight the seriousness of divorce for children, and that taking the wife back and settling the matter with her might be better.