In the PC version if you click on an adult passive mob with it's respective spawn egg a baby is spawned in. (Cow Egg on adult cow spawns baby cow)
In the PE you damage the parent by clicking on them with the spawn egg.
I did see that using eggs currently produces babies roughly 50% of the time so this might no be a bug, but I thought I'd bring it up just in case.
Please do not assume the differences between any versions of Minecraft are bugs. If you wish to request new features or tweaks for Minecraft: Pocket Edition, please use the Suggestions Forum or Subreddit.