Is the gate drive on your high side switches going at least 10V above the drain voltage? This is a requirement for using these devices.
You can get the following information from the datasheet: With 10V Vgs, the ON resistance should be 40 milliohms max. With 10A passing through the transistor, it will dissipate up to 4 watts. The thermal resistance from junction to ambient is 62.5 deg C/W. At 25C ambient, without a heat sink, the junction (chip) temperature will reach 25+(62.5*4)=275C. The maximum allowable junction temperature is 175C, so the device will be destroyed without a heat sink.
How much heat sink do you need? I would try to keep junction temp below 125C, for greater reliability. This means you need total thermal resistance from junction to ambient of less than (125-25)deg/4W=25deg C/W. Thermal resistance from junction to case is 1.9 deg C/W, and thermal resistance from case to sink is 0.5 deg C/W (I think this includes a thermal grease of some sort). The heat sink thermal resistance must therefore be less than (25-1.9-0.5)=22.6 deg C/W. Heat sinks are specified for thermal resistance.