I believe the viewing of the individual as he/she is as foundation of a nation is a harmful way of trying to keep a nation stable, sustainable and strong.
Parents are willing to sacrifice their own interest for that of the future.
So why not have that kind of instinctual mentality influencing the source of political legitimacy.
The facts that only NFP can vote would show different voting patterns from what we see now.
Voting preferences would be geared more towards Sustainability, as their children will inherit the nation.
And as such NFP's have more of in interest in the welbeing of the nation.
I am not a parent, and I would like the right to vote. Yes, there are some self interested voters with no concern for American's future (ex: most Clinton and Sanders supporters) but there are plenty of non parents (ex: myself) who actually do put the interests of our nation ahead of our own.
Well frankly, this excludes the innumerable opinions of individuals (those who might not marry or raise children). It would be daft to assume that individuals cannot take into account the benefit of the nation, opposed to their own. But regardless, the fundamental purpose of voting is to express the opinion of the -entire- nation (assuming the person be 18 years or older).
We look for more than a mere "stable" nation. We seek a nation of innovation, which quite frankly, is impossible in stagnation. It is ultimately the individual who determines the degree of this.
Speaking from my own town, I know a majority of parents who would vote for any party that promises tax cuts and strong public education, and nothing more. And a majority of grandparents who would vote for any party that promises pension increases and strong public healthcare, and nothing more. All voting is self-interested to some degree, the only difference is what your self-interests are.
Not to mention that this idea is incredibly non-democratic.