Problems with dc motor driver design: Mosfet fail
One huge issue I'm seeing is a lack of any flyback diode for the motor. You need to add a high current diode from the mosfet drains to battery positive. This will give that energy somewhere to go. You'll want to connect the diode physically close to the mosfets (the motor wires will produce their own small inductive kick). This is probably the reason your mosfets are dying.

You'll want to read up on how to pick this diode, but the key parameters are current (a good rule of thumb is to handle as much as your motor draws), and reverse recovery time (I'd go with a schottkey).

A second problem is that you're driving the mosfets at only 5v Vgs (slightly less actually due to the resistors). Looking at the current/Vgs curve in the datasheet. The Rds will be approximately 0.25 ohms. In parallel that will be 0.125, so at 10A you're looking at 12.5W of dissipation. That's quite a bit.

The third problem is that you're driving the mosfets from the arduino directly. Since the arduino can only supply 20ma or so to the gate, the switching times will be slow. This means more power dissipation, especially as the frequency increases.

You can solve problems 2 and 3, by using a gate driver IC. This will drastically improve the power handling of your motor driver circuit.