Pathfinder Adventure Path

Spiked Pit Trap (CR2)
Type mechanical; Perception DC 20; Disable Device DC 20

Falling Damage (10ft): Automatically 1D6 (Acrobatics check DC 15 to avoid)
Secondary Damage (Spikes - 1D4 spikes per target): attack +10, damage 1D4+2 each spike (Reflex DC 20 avoids spikes)


Personally, I would set this up as attacks.

Then have it play out in the following way:
1) Have player roll Acrobatics to see if they avoid damage from falling.
2) Have player roll Reflex save to avoid spikes. If the player fails... have the player (for dramatic effects) or you can roll in the Core Dice the number of spikes that they land on.
3) Use Falling Damage attack and confirm damage if the player failed their Acrobatics check.
4) Use Secondary Damage (from spikes) attack and see if any hit the player using the Declare Attacks and Resolving Attack Actions if the player failed their Reflex Save. Simply click on the number of attacks the spikes get to make on the Declare Attacks window.

 

You as a GM can change any rule as you see fit to make the game experience more fun, realistic, or to speed up game-play. The official ruling is below, if interested.

Treat each spike as a separate attack is the "official" ruling, since: 1 even while a creature is falling they still can use their dexterity to somewhat control their descent (not flat footed, think sky divers); and 2 each spike at the bottom of the trap are positioned in a different location. The 1d4 amount of spikes is the number of spikes that do hit, however, it does not constitute instant damage since each spike need to attack and pass the falling creature's Armor Class.

3.5 OGL http://www.d20srd.org/srd/traps.htm

Code:

Pit traps often have something nastier than just a hard floor at the bottom. A trap designer may put spikes, monsters, or a pool of acid, lava, or even water at the bottom. Spikes at the bottom of a pit deal damage as daggers with a +10 attack bonus and a +1 bonus on damage for every 10 feet of the fall (to a maximum bonus on damage of +5). If the pit has multiple spikes, a falling victim is attacked by 1d4 of them. This damage is in addition to any damage from the fall itself.

"The victim is attacked by 1d4 of them"

Pathfinder OGL http://paizo.com/pathfinderRPG/prd/environment.html

Code:

Pit Spikes: Treat spikes at the bottom of a pit as daggers, each with a +10 attack bonus. The damage bonus for each spike is +1 per 10 feet of pit depth (to a maximum of +5). Each character who falls into the pit is attacked by 1d4 spikes. This damage is in addition to any damage from the fall itself, and the statistics presented above are merely the most common variant—some traps might have far more dangerous spikes at their bottom. Pit spikes add to the average damage of the trap (see Average Damage, below).

"Each character who falls into the pit is attacked by 1d4 spikes."