Hi, I'm Dr. Bonk. I'm sorry to hear about your lamb's troubles. It sounds like you're lambs are affected by either white muscle disease or copper deficiency.
White muscle disease is a lack of selenium and vitamin E. It can show up right at birth or within the first few months of life. Generally these lambs start out normal and then slowly deteriorate with ataxia, muscle stiffness and then eventually succumb to pneumonia or starvation because they can't move around.
Copper deficiency can also show up at birth or develop within the first few months. This usually causes neurological problems eventually causing them to not be able to use their legs.
You would definitely be safe to give them a dose of Bo-se, and if you haven't already, give the rest of the lambs a dose too. Most of the United States is low in selenium meaning that animals don't get if through their diet. So they need alternate sources. Injection is by far the easiest because you know exactly how much and who is getting it.
Also consider a copper supplement, especially if your ewes are not on a copper supplement. There are several types that can be added to feed or to their minerals. It would be best to look for an injectible option for these affected lambs to get it on board quickly. You should be able to find something at most feed stores, if not then a veterinary clinic.
Also, the affected lambs may require nursing care. If they are not moving around enough to eat or nurse, make sure they have feed and water within reach. You may want to move them into a stall or other enclosure to keep them safe and away from the elements until they are feeling better.
With both of these issues, supplementation can usually stop the progression, but there may be lingering effects that won't go away. Each lamb is different in their reaction.
I hope this helps. Please contact me with any further questions you may have. Thank you and good luck!