Is performing a two-rak'ah prayer as a thank you to Allah for the complete creation of four children an innovation (bid'ah), and should one continue it or stop?

Prostration of gratitude (Sujud al-Shukr) is sunnah upon the renewal of apparent blessings or the دفع of misfortunes. Two rak'ahs should not be prayed instead of it, as there is no reported evidence for that. The Prophet, peace and blessings be upon him, prostrated when something pleased him, or upon the conversion of a tribe to Islam, or when his intercession for his Ummah was answered. The Companions likewise prostrated upon the removal of misfortunes or the attainment of blessings. This prostration is performed for this specific reason only, not for every blessing, as the favors of Allah are countless.