I am concerned with the age of your children. First I did a search on you and find you are a new user only in the black seed forum. Could it be you are under disguise and that is ok if not then I want to suggest that you check on a few other forums. Such as Dr. Mom, Dr. Hardy and Dr. Sutter. I want you to know that we are not doctors here and I would highly suggest, number one you eliminate any health risk. So could the problem be more than just a general well being issue.
Two, I would suggest that there are many children underweight but we need to look at the whole diet. So exactly what kind of oil are you giving, send me the link so I can look at the oil. How much juice are you giving and most important it must be one hour before breakfast. At breakfast what are you feeding them? Are they still on breast milk? etc.
We cannot give out medical advice but up on knowing the whole problem we can advice what point of action would be suggested.
Anyone who gives out medical advice would be in trouble so I suggest you be very careful with your children's life. Lack of sleep could be many problems. Anything from being frightened in early life, to having the wrong diet to eating two hours before bed. No one should have protein or carbs before sleeping. Are they having meats of any kind of just baby cereals and milk. Any child on breast milk should not be small.
So I have many questions. See what you can tell me and go ask many doctors. Let me warn you that most know nothing of blackseed so they will not recommend the seeds but maybe they can tell you exactly what is wrong with your children.