What should I do now if I feel guilty for not giving money to a little boy who asked me for food while I was busy, and I am afraid of how I will be held accountable in the hereafter?
You didn't ignore the child out of bad intentions, in fact the mere thing that you are thinking about it, shows that you want to help the needy in future. So ask Allah swt to help you next time do what is right in the appropriate time. As it is famous when one intends to give charity, Satan interferes and tries to prevent him. May Allah help us all do the right thing in its right time and way.