What is the ruling on supplicating with: (O Allah, verily You are Pardoning and Generous, You love pardon, so pardon me, and my children, and all believers) in prostration?

It is permissible to supplicate during prostration with whatever one desires from the good of this world and the Hereafter, because it is one of the occasions when supplications are answered. The Prophet, peace and blessings be upon him, said: "As for prostration, strive in supplication therein, for it is most likely that your supplication will be answered." The aforementioned supplication is good, and it is permissible to supplicate with it during prostration.