Limit internet speed for a specific device or profile?

With XFi I can do some types of limits and controls on a profile, like time and content. I would also like to limit internet speed (bandwidth) for a specific profile. For example, let's say I have a profile named "Kids" with a couple of devices assigned to it, and I would like to limit the bandwidth of that profile (or the devices in it) to 20 Mbs.  Is that possible?