Is my marriage, which was based on previous doubts about religion, invalid, and does that affect the legitimacy of my children, and what is the ruling on renewing the contract?

Mere doubt about disbelief (kufr) at the time of the marriage contract does not invalidate the contract, for the fundamental principle is that you are a Muslim, and Islam is not removed by doubt. Your marriage is valid, and your children are lawfully attributed to your lineage. There is no need to renew the contract. We advise you not to indulge in obsessive thoughts (waswas).