Rubber mulch will get hot in the sun, just as anything in direct sunlight will heat up. The black rubber mulch tends to be hotter since this color absorbs more sunlight.The lighter the color, the cooler it will be in the sun. Shoes are always suggested and recommended. The advantage to lose fill rubber mulch is that when you kick the loose fill rubber around, it is cool underneath.  Rubber mulch does not conduct heat. It will be cool just below the peak. Keep your shoes on and get out and play.