So sorry to hear this and I appreciate you coming here, but this is a very serious problem and I must suggest you seek a specialist. You do not have a firm diagnosis and this is your child who at this age can not tell you what is wrong.
Now black seed will help with cardiovascular problems in adults but what I see from your post that this heart problem may be a side effect from the real problem. Slow development may be a sign of other serious problems.
children should take 1/2 the dose of adults and if you choose to try nigella, it can not hurt. You have to consider Nigella sativa as a preventative or vitamin in effect. I would not suggest you take Nigella and forget to find out the real cause.
With searching the internet I find many studies indicating Nigella sativa has been shown to improve congenital problems in children. The perfect dose is 1/2 teaspoon in a small glass of freshly squeezed orange juice or any juice your child like, such as fresh apple juice. Do this twice a day. I am talking about 1/2 teaspoon of the best oil you can find.