there are several of these in my hothouse, in tubs and they are eating my seeds before or after they have started to sprout - mainly carrot seeds and nibbling around the base of celery - what do I do?
Roly poly bugs don't generally eat living plant material or seeds - they are part of nature's clean up squad and primarily consume decomposing or decaying material, and will only start nibbling on live plants if there are too many of them.
Clear away any dead plant material, general debris, rotting wood (including very decomposed wood chips) which is in or around the pots, and keep the whole area in your hothouse as clean as possible. If the potting soil you used is not a good one and is full of partially decomposed material,that will attract them too. Diatomaceous earth will help if there are a lot of them - further suggestions for dealing with these here https://www.planetnatural.com/pest-problem-solver/lawn-pests/sowbug-pillbug-control/
Three things come to mind - assuming you don't want to just spray toxic chemicals. First you could sprinkle diatomaceous earth on the soil around your seeds and plants. This will repel and/or kill through causing micro-abrasions in their exoskeletons leading to desiccation - at least in theory.
Another option might be to provide an alternative, more attractive food source for them in a location away from your plants. My son and I keep a terrarium with live plants, millipedes, and two varieties of isopods (roly polys). The only time the isopods bother the live plants is when we don't feed them. A slice of apple or a blanched piece of zucchini always draws them out of hiding to feed.
The third, which would be more labor intensive and might not completely solve your issue is to manually remove them. You could put out a dish with some tasty treat for them (see above) and then just empty the plate outside once the bugs (crustaceans) show up to feed.
They're actually crustaceans, there must be a lot of water nearby or very damp saturated soil around. They're also called pill bugs. You may have a drainage issue - fixing that would make it dryer and uncomfortable for them. It could also be too much water from somewhere else, maybe a gutter downspout nearby, or just over-watering your gardens.