Slaters (also known as roly polys) are land-living crustaceans that commonly feed on decaying matter and help to build soils. So while they can be considered friendly under many circumstances, in certain conditions when their food sources are scarce, they can turn their attention to your young seedlings. Sometimes it's just a matter of a seedling being in the wrong patch at the wrong time.
It is believed that the warm, arid climate in conjunction with dusty, dry mulches, have provided the ideal breeding ground for slaters to become something of a nuisance. But more recently we have noticed that even in wet conditions, slaters can be problematic too. Heavy rains resulting in decaying plant matter - their favourite food source - will attract slaters in large numbers. Usually seeking cover under the mulch (or nearby dry areas such as under timber, pots, bricks etc), they pop out when safe to feed.