I bought these for my 6 month old daughter thinking they would be the most comfortable with the screwbacks.  They can be tricky being so tiny to screw onto a squirming infant but once it they stayed put.  The only problem was after being in for 3 months I noticed irritation and blood behind one.  There is quite a bit of room where they can move back and forth.  They don't sit tight against her ear.  Im not sure if it was pulled or what happened.  I changed them back to the piercing studs and the problem resolved.  They are cute and secure so I will change them back someday.

8/2013: Update. Did change them back and they have since fallen out and been lost.  The back is so tiny its hard to get on.  I ended up buying piercing studs that have a rounded back and are the only earrings thats have not fallen out.