What is the ruling on one who borrowed money in childhood and forgot its owners and amount? Is it permissible to give it as charity on their behalf with the intention of repaying the debt? And does anointing oneself with olive oil invalidate ablution?

The basic principle is to return the money to its owners. If you have forgotten them, then give it as charity on their behalf. If it is impossible to identify the owner after thorough investigation, then give it as charity and spend it on the welfare of Muslims. Strive to estimate the amount of money to the best of your judgment. And anointing yourself with olive oil does not invalidate ablution.