If you try to give up sleep, it will make you less alert around your kids. Aside from potentially exacerbating a dangerous situation should one occur, it will also make you less responsive to them, less playful, and less patient. They will eventually learn to just leave you alone -- even if that's not what you intend -- and that will weaken your bond with them and make them less communicative and more rebellious later on.
I hope this isn't coming across as a lecture or a moral crusade. I'm not implying anything about how you are as a father. Just that, if you have a full time job and two kids, then you really don't have the time left to spend on personal projects or freelancing without sacrificing some of your relationship with your kids. The time you spend on freelancing or personal projects could instead be spent reading to them, playing with them, doing cool science-y things with them, and those things are all really important.
If someone had discussed this with my father while I was still young, I might still have something to say to the man.