Not sure what waterproof liners you mean, I've never heard of that.

But generally speaking yes, you do need a waterproof cover (though the bedwetter pants are for older children who still a diaper at night, they're a diaper and cover in one that they can pull on/off themselves, not a cover) for any fitted diaper. The outside of the diaper gets wet pretty much automatically (they're basically like a towel, if you pour water on it, the underside will get wet even if the towel is very absorbant or even folded in half) so you need the cover to keep clothing from touching the wet diaper. Newborns aren't usually heavy wetters it's true but the outside of the diapers still pretty much always get at least a little wet.

Karen.