leaving just one dorf isn't going to be a good way to get more children. The best thing would be to pair up the starting dwarfs in male/female pairs, have them work and hang out in "couples" burrows. Give them plenty of time off too. This will give them the best chance of pairing up and having kids. Don't kill off the remaining odd one out, because one of your dwarfs might be gay, and you'd need a replacement breeder. If any haven't paired up after 1 year, switch the burrows around of the singles. Any dwarves who get married to the same sex go into the atom smasher (not being homophobic, just to optimize the adults/kids ratio).
If all goes well, each pair should produce 1 child per year, or 3 children / year and it will take 12 years for the first ones to become adults. So you'd have 6 dwarves in pairs, with 36 kids. Once the first kids grow up, you'd have 3 kids reaching adulthood every year, so expect ~3 breeding couples to form every 2 years under optimal conditions. If you had 120 children, an average of 10 would become adults every year so you'd need 20 adult breeders to maintain the numbers, so you'd be looking at a 70% child fort. It wouldn't be so spot on though, as you'd need to only kill off older dwarves when you could be sure of new breeding pairs starting to produce. You'd have to maintain 10 breeder pairs and kill off all older or surplus dwarves.
So, you could carefully groom a fort with over 100 kids, then kill all adults but one and enjoy 1 year of your kiddy harem. But after that the first batch will grow up and you'd have to kill them off or reinstate breeder pairs, so this would actually cut into the child/adult ratio in the long run.
Btw, if you're interested in dwarven genetic engineering/eugenics, it looks like DF only looks at the mother's DNA at least for animals, so you'd want to select breeder mothers based on stats and breeder fathers isn't important.