TL;DR: No, you cannot always guarantee that even two Inhumans will have an Inhuman child, but it's likely. And simply having the Inhuman gene does not guarantee Inhuman abilities.
It's important to note that Inhumans are simply humans who carry the original genes added by the Kree millennia ago. Unlike the X-gene, which guarantees a mutation of some sort if active, the Inhuman gene merely makes a mutation possible when (and IF) the subject is exposed to Terrigen Mist. Conceivably, an Inhuman could live out their entire lives as a human, never being exposed to the mist, and never knowing their true potential. Granted, however, that even without the mist exposure, a person with the Inhuman gene would be a little faster, a little stronger - not enough to appear superhuman, but certainly enough to excel when they apply themselves.
To your question, however, there is always an amount of uncertainty when it comes to genetic heredity & selective breeding. If two Inhumans produce offspring, the odds are certainly good that the offspring will be Inhuman as well. If an Inhuman and a human breed, it would depend on whether or not the Inhuman gene is dominant, which is something that we don't know. Potentially, at least, either pairing could result in a human child.
UPDATE: The events of the Infinity arc revealed that there had been unknown tribes of Inhumans breeding with normal humans throughout history. These pairings resulted in both human AND Inhuman offspring, with the Inhuman descendants not realizing their potential until Black Bolt
detonated the Terrigen Bomb, thus activating their dormant Inhuman genes. Thousands of people all over the Earth fell into Terrigen-activated cocoons, emerging later as fully transformed Inhumans.