The Grunt's B-day skull is pretty easy to get. First you beat the game on any diffuculty level, but easy.Then go to the second level on normal or higher. Go through the level normally till you get to where the drones are passing back and forth throught the pipes(right before you jump down to meet the Arbitor).Stop right where you're supposed to jump down to the Arbitor and look down. Right below you , you will see a green arrow pointing backwards. Jump on the arrow and turn around. You will see a hidden room with the skull in the middle.

The skull's effect is when you hit a grunt with a headshot it will burst into confetti and children will scream YAY in excitement.
(Note: Since this skull is a silver one you will not unlock an achievement)

[i][b]ADMIN EDIT:[/b] Assigned the spoiler (h3) tag. Please assign the proper spoiler tag when posting spoilers-- as a category when posting a forum topic or a blog/story entry, or the bbocode spoiler tag when posting a comment. Thanks![/i]