The CPU IRQ needs to register the device. The maximum transfer speed and electricity usage of the parent, will be your limitations, because that is what interrupts the CPU.
English version: You can use a USB 3 hub on USB 2, but you'll only get the max speed of your USB 2.0 controller. USB 3.0 devices used, will have to be USB 2.0 compliant.
If it's a powered hub, then you might be able to draw more power from the 3.0 hub for 3.0 devices. But transfer speed is still USB 2.0.