Why can't I ever have dragons....
Give them the [SEMIMEGABEAST] tag. It will make them much more common, and ensure enough breeding pairs survive worldgen. I did it with hydras because they kept getting killed due to being so trigger happy.
Claerly you are unfamiliar with emergency modding.
Changing creatures requires generating a new world for the changes to take place.
Only adding/subracting creatures (or any entries) themselves requires a regen. Like SRD said. Almost any other changes to pre-existing creatures will take effect immediately, including adding the [CHILD:x] token mid-game.
The path you need to go to in order to accomplish your breeding program is [DF directory]/data/save/[your region name/number here]/raw/objects/creature_standard.txt.
If you use [CHILD:5] or something, hatchlings will be considered adult and begin laying eggs of their own in 5 years after hatching. they simply won't be full-sized until they're 1000 years old
Remember; the raw files in "[DF directory]/raw/objects" do NOT affect worlds you've already generated. The raw files there are only used to generate new worlds with, while the raws in the save's own raw/objects folder affect that save alone. This is a significant improvement over the days of 40d when all saves used the main raw/objects folder, so if you wanted to remove something, or use a mod that removes something, older saves would cease to function because the game [still] can't handle the loss of a raw entry mid-game, because the save calls on that entry.