You can be an entrepreneur at any point, you can only be a father to these young children once. Relax and enjoy your children, especially whilst they are very young. My kids are 6 & 8 and with school, sport, general tiredness that they get, I now have the time to do side projects and the like. I tried when they were younger but at the cost of some quality time with them - and with my wife.

If you really do have to do something then I found getting up at 5 and having 2 hours before the rest of the house was too awake (my kids sleep in) worked pretty well, but you have to remember to go to bed at a decent time.