I don't know the full list of vegetation which is poisonous, but an obvious one is laurel - we lost a ewe and had another sick for a while after they escaped into the garden and chewed on the laurel. The bush has long since been cut down and burned and the stump dug out. Rhododendron is also poisonous to sheep and I would think that rhubarb and daffodils are too. Because your lambs are orphans you need to be extra careful as they would normally learn from their dams. If you can face looking through the DEFRA website there might be a list of plants poisonous to sheep. In addition to plants which are actually poisonous, there could be some which would cause young lambs an upset stomach. You are probably best to limit them to the lawn itself, as long as you haven't used any chemical herbicides and so on on it.