I started a new world with myself married to a guy, with no children. So, to populate the village, I started having children! However, none of my children had assigned 'jobs' after growing up; I can't hire them to do anything. (i.e. nothing appears under the 'special' menu). I had a whole bunch of kids, finally one of my grandchildren was born as a Cleric and I was able to use the Cleric commands with that one (after it grew up).

Do I need to wait for grandchildren and great-grandchildren to have hire-able villagers, or is this a bug?

Your children have abilities under the "Work" tab while interacting.

Grandchildren are regular villagers who can later be hired.

So, the 'job title' designation and/or number of hearts of the villager does not seem to affect their hire-ability. All of the villagers are my own children, grandchildren, or generated guards.

More info: In a nearby generated Minecraft Town, I was able to hire any of the people there, except the guards (I guess guards are not hire-able?)

Here are a couple of screen shots of my own grown child, unable to hire:

Screen Shots of a villager titled "Villager", unable to hire:

Screen shot of a villager titled Fisherman who I AM able to hire (caught her blinking, whoops)

Screen Shot of an "Armorer" villager, unable to hire(blinking, lol):

Screen shot of a guard, unable to hire: