Hall effect output voltage
Since you just want to operate LEDs that designate an on / off indication you may not even need to use an op-amp for this as suggested in the comments. Instead setup a circuit that uses a comparator device. These can be found with four per package. Set one input of the comparator to a reference level of say 350 mV. The Hall sensor output feeds into the other input of the comparator. The output of the comparator can drive the LED with suitable series resistor. What the comparator will be showing is if the Hall sensor voltage is above or below the reference voltage.