You could compare the magical critters to say, computers. We as people, can only create machines that think so well -- science and engineering give us cars, trains, automobiles, and multiprocessor computing machines. We have biology that permits us to treat and prevent a variety of different diseases and even engineer a few simple ones.
Figure that the capacity of the "created" magical race is limited by the "creating" race, unless you're allowing for a lot of time for "evolution" of a sort to kick in -- the race that created warforged probably didn't expect them to achieve sentience over the course of the Last War, but about what level do you think they had to be to dream them up and manufacture them?
You might treat them like the Leadership feat -- generally speaking, you have a minimum level of 6th to take the feat, and your cohort is about 2 levels lower than you (if you're really Charismatic and meet a variety of other requirements). A safe bet might be in making the creators about twice the level of the creation.
A 10th-level golem for example, might be created by a 20th-level mage. You can modify this one way or another based on how many mages (or other archetype) are responsible for the creation -- as is probably the case with the warforged example. Many, many magewrights probably collaborated under the guidance of a couple higher-level artificers.
The more commonplace (and subsequently low-level) the race is, the more powerful the creator ought to be -- or the more time ought to have passed. Generally speaking, you could assume that "time makes more things possible," and while a lone half-fiend might be a CR 8 and uber-powerful, with successive generations only certain traits might be passed on, until only the most immediately-useful-for-survival traits stick with a CR 1 race (like tieflings).
Ultimately, those couple "plus-twos" to a couple ability scores will be the most useful -- and then you have a new race with a unique power. Maybe the tieflings were started by CR 20 pit fiends, but with a couple centuries they've become widespread in the world.
So yeah, consider how much time has passed since their creation, and how widespread they are.
--Nick