Most of us were taught to start our IV's bevel-up, but when you're trying to thread a plastic catheter into a child's vein, quite often the leading edge of your trocar pierces the opposite wall of the vein before you're ready to advance the catheter.

To prevent this, try inserting your IV catheters bevel-down. Doing so will make the angle of insertion much more shallow, thus minimizing the chance of poking the very end of that needle through the opposite wall before the tip of your cannula enters the lumen of the vein.