I decided to keep bottle feeding my baby buckling ( I bought him a few weeks ago as a bottle baby), milk for a while longer. I am currently watching my chunky monkey of a little girl (about a month older than him) barely stopping the nursing and she turned out so silky and healthy looking, and I think the milk is a really good, healthy thing for them. As long as I enjoy it and he does too, and supply isn't a problem, I will probably let him keep having his bedtime snack for another few weeks yet.
As long as they aren't having problems with worms or cocci (definitely check those first)....you could give them milk if you think it would help them, it certainly won't hurt them. The enzymes and probiotics etc, would be good for them. That being said tho, nubians are all pretty lean as babies. When Bambi came to us at 6 months, I think it was, she was scrawny, all bones and skin. But she ate fine and was actually healthy. I don't think I have seen a nubian baby that wasn't skinny. Some nubian lines are even slower to mature as well. Your kids are likely just taking time to adjust...It has only been 3 days right? Maybe once they aren't so lonely for mama they will focus on food a little more.