'going to' means that you have planned it. It is something that you have thought about and planned. E.g. I'm going to clean the car at the weekend. present continuous for future means that you have arranged something for the future with other people. You know about it and they know about it. E.g. I'm meeting Jane on Saturday, You have spoken to Jane, she has agreed to meet you, you both know what time and probably where to meet. 'Going to' does not have this idea of arrangement with someone else.

Saying for example "I am going to" usually indicates an intention for the future such as "I am going to make dinner tonight." The present continuous shows that something is happening in the present moment, and it as an ongoing actions, such as "the children are sleeping." I am not sure that "[be] going to" can easily indicate the future, unless you say "they are going to be sleeping," which would mean that they will be sleeping (usually as an ongoing event) when XYZ event happens. This is similar to the French futur anterieur.