@Bumpitybumper
Sorry if I misjudged your seriousness of opinion on this - I was just thinking absurdly of the potential place that this could end up.
The whole idea of the vote is that everybody deemed old enough (and with capacity) to make an informed decision is able to do so. The accepted way that we do this is that, once reaching 18, you get to vote in every election there is until you die. Even the older people whom you appear to resent having their democratic say were not allowed to vote in the first 18 years of their lives. They didn't just appear as old people: they were children and teenagers once, too, and their parents had to make voting decisions with their well-being in mind as well. Children and their needs are indeed very important. The exact same is true of the elderly, and everybody else in between.
Your suggestion (albeit unintentionally) gives fuel to the ill-educated arguments some people make when complaining about people who have a lot of children - all of them 'a massive drain' on the NHS and the education system etc. etc.; whereas each person who is born will individually cost the state a lot of money throughout their lives and, theoretically, also individually pay a lot of taxes into the system throughout their lives - it's completely immaterial how many other children their parents had, as the emphasis is on the individual.
Like it or not, except for a very, very tiny minority possibly, everybody votes selfishly - it's what suits ME and/or MY family the best (because people don't tend to have children altruistically for society - they have them because THEY want them).
Why should those who have lots of children because they want lots of children (as is their absolute right) then get to dictate policy unfairly, based on what THEY believe is best for their children and all children in general (which will not necessarily be the same as other parents believe - 'Parents' aren't just one great big homologous group)? How do we even know that their children will grow up and agree with the voting decisions that were made on their behalf? Could we see court proceedings and irretrievable family breakdowns in extreme cases?
And why just children? What about, say, terminally ill people? Should they get more votes because they won't have as many future voting opportunities as healthy people - or, conversely, should people be banned from voting when they're not expected to live until the next election? How about elderly people in general? Should a 90-year-old be disenfranchised because they won't be around to live with the consequences of long-term decisions for which they voted? Maybe we should only allow people between the ages of 0-70 to vote, or 0-40. Or even ONLY children: once you reach 18, you lose the vote that you had from birth.
Sorry, I'm still not convinced at all.