This "problem" solves itself when you think ahead to the fact that children will have the ability to vote for themselves when they become adults. The simple act of raising a child to voting age ensures that you have increased your family's voting power, if that is your concern.
You know who else has a quantitatively bigger stake in the future of the country? Those with more money and property.
If you nominalize by capita, people with children have less of lots of things. Fewer cars, less property, less income, lower alcohol consumption.