Demeny voting is giving children votes to be used by their parents, until the children reach voting age. The interests of children are not always represented well in democracies and giving their interest some weight might lead to better policies.
Older people vote more often. And thus their policy preferences are more likely to be followed than those held by young people. No one under 18 votes so their policy preferences get very little attention.
But why would you think parents know what policies are best for their children? We trust parents to pick food, bedtime, schools, clothes etc for children and in comparison to them picking a political candidate occasionally is minor.
If giving children votes would result in more money for schools, less money for pensions etc then the people who want less money for schools and more money for pensions will stop it happening. Which means we should argue on the morality of giving them votes not the particular policies that are likely. Women supported alcohol prohibition more than men did. But women's voting was still the right thing to do even if some policy choices they initially supported were wrong.