Some people don't use knee pads while wearing paintball pants, but I'd recommend buying a good pair of them if you're planning on taking your slides and dives seriously.
100% wear paintball knee pads under my pants, The way paintball is going these days the players want light weight everything that means removing the heavier parts of the pants (The Protection)
Totally agree. Always wear knee pads under trousers. Depends what and where you play you would be amazed how much time you spend on your knees shooting or taking cover.