I had a bit more than 21 doors, but since I couldn't be sure if they were all registered/valid, I just added a large house with 18 more doors in the center of the village (with cured villagers running around) and also 5 new doors to existing houses.
Here is what happened:
I did see my cured villagers facing each other with red hearts above their heads about 10 times in the last 30 minutes, and I finally got one child. Just one.
4 of my 6 original cured villagers vanished. I can't find them anymore.
The only child I got became an adult and had two other children with one of the two remaining cured villager.
I also need to mention that before having 6 cured villagers, I ran the game for about 5-6 hours with only 2 of them roaming the empty village, and nothing happened at all.
In snapshot 13w19a, the cured zombie (still?) do not breed.
I have 6 of them in my village and no regular villager at all, and I also have plenty of doors/houses, but I didn't get a single children in hours of gameplay.
However I haven't seen one despawning yet.
As a side note, it seems to me that cured zombie are running a bit too fast.
In my villages, I always plan a trap (usually a 2x2 hole in the ground) in the north-west house, with an underground river pushing the villagers to the south-east corner, where they can escape. They still don't pay too much attention to the other houses, but at least I can see them running across the village, heading back for the north-west corner 🙂