Now that you mention it, having kids waiting around for hundreds of years to inherit anything might not be a good idea, since eventually at least a few will turn out both greedy and impatient. I guess it would depend on how likely the child of a dragon knight was to develop magical abilities and have an aptitude to become a dragon knight, as well as how many dragon knights were actually needed to rule over the available lands. If the children of dragon knights were generally powerful, it could cause problems with either limited opportunities to become a dragon knight, or limited opportunities for advancement once they were dragon knights. On the other hand, if magical abilities were not fairly hereditary there would be a limit to how many children dragon knights would be willing to have, if a significant portion would be limited to the much shorter life span of a normal person.
Elves in Rivellon have much longer life spans than humans (mentioned, but not expanded on significantly in Divine Divinity), which may have been part of why they were chosen to be the first dragon knights.
