Maen nhw angen cwsg or Mae angen cwsg arnyn nhw or Mae arnyn nhw angen cwsg or similar.
How about "They need sleeping" (don't even know if that's something you would say in English)?
It sounds wrong to me in English.
"The car needs cleaning" can work, because we can clean the car, so some cleaning has to be done to the car. But you can't sleep a person.