You can worm the ewes it won't affect the lambs. I think the recommended is to worm ewes at 3 weeks and 6 weeks post lambing, and the lambs at 6 weeks, I wait until the youngest lamb is 6 weeks then do all the lambs together. I do my ewes whilst they are in mothering up pens the day after lambing. I don't move onto new pasture once wormed, I save the grassiest field for post lambing and keep them there for a few weeks.