Its hard to determine without knowing what kind of router your using to determine weather it has QoS service in it.
I have the same problem as you with our 11 year old, PS3, youtube and our Iphones\ Ipad. It bogs me down very quickly and gets annoying very fast. Using the LAN ports and Mac addys I QoS'd all the devices and now I can do my own thing on my comp with the PS3 playing COD and my wife upstairs streaming into her IPad, unlike before all I could do is enjoy the lag because they were hogging all the bandwidth.
Here's a pic of how I set up the QoS in my netgear R6300 router and it works damn fine.