I disagree with you categorizing it at 50% - seems like an arbitrary number that is unrealistically high in addition how do you measure productivity? Time at the desk? As well a lot of non children workers are also equally not as productive so I think you have a straw man argument going on here.

Finally, you miss the truth to the matter. If you are getting paid in equity you want the whole boat to float so short term mindsets such as working less for a couple months vs keeping the whole ship floating as a whole is more important. Imagine all the companies with parents just burned out right now - that's the difference.