Disregard the below information, You need to gift one of the 2 out of the couple a cake (it actually takes 2) and then they will have a child.
You just need to wait. For example, TheDiamondMinecart (DanTDM) was playing Minecraft Comes Alive and married his son to a villager. Soon, his son was carrying a baby around, but it never grew up. Weird. Dan called it a "phantom baby". This might be a glitch, but the children (once adults) DO have babies even though it might seem like it takes forever. Just sayin'. But you can give each of them on cake (You need two cakes in your inventory and gift one to one of the couple)
TheDiamondMinecart's YouTube channel: https://www.youtube.com/channel/UCS5Oz6CHmeoF7vSad0qqXfw
If the link doesn't work just copy paste it.
Hope this helped.
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