Is it permissible for someone who gave a loan and did not intend it as charity before the repayment of the loan, to then intend a portion of the money as charity on behalf of himself and his parents, and another portion on behalf of his newborn child with the intention of shaving his hair and donating its weight in gold, all before the repayment of the loan to its owner?

If you intend to include your father in the reward of the charity after it has been paid, the matter is subject to scholarly disagreement among jurists. Some hold that the reward is only obtained with a prior intention or an intention concurrent with the act, while others permit it even if the intention comes after the act. If it is said that it is not valid to include him in the reward after giving the charity, then you can dedicate the reward of that charity to him by supplicating for him.

As for weighing the newborn's hair and giving its weight in silver or gold as charity, it is a recommended Sunnah (definition) according to the majority of scholars, while some consider it permissible but not a Sunnah. The intention for this specific Sunnah charity is not valid after it has been given; rather, the intention must be made before or concurrently with the act.