They are fast. Almost as fast as a running player. They also attack slightly faster than adults.
They can blend in well with a horde of zombies. TNT is quite good, but somehow they can hide in holes and wait to ambush you.
They are too small. Many players tend to go shoot the babies first when a lot of mobs are coming, but will often miss and allow the mobs to close the distance. Furthermore, once they are close to you, your screen will shake everytime they hit you while you have to bend down and hit them as their hitbox is lower than you.
Recommended weapons:
Power V bow and healing arrows for an insta-kill.
Sharpness, fire aspect and knockback 2 for keeping the distance while also setting them on fire so they are more visible.
TNT landmines with 2 block deep hole. The Baby will die instantly.