What you want is a Bass Shaker Tactile Transducer
Bolt that onto their bed frame and they will wake up.
On a maybe more serious note, just spend like a moment of your time and look at vibrating alarm clocks. There are a gajillion of them out there. People who are deaf use them. There was a deaf guy in our dorm building and on some mornings his vibrating device dropped to the floor and made a ruckus on the floor waking a lot of other people up.