I am luving those little babies, they are toooo cute.
I had a nubian cross give birth to twins last year, a big boy at 10 lbs and a little girl weighing 7 lbs. As they "grew" she was always on the small side, and still is.
Added a fullblooded nubian doe last Fall, the same age as my nubian cross doeling and the two of them are about the same size, smaller than your average nubian.
Looking at the pictures, your kids appear healthy.
Cute kids, love the colors! I wish LaManchas got crazy moonspotting like that.
If they're not growing, and also looking a bit thin, another thing to check for would be lice. It's amazing how goat lice can just suck the life out of kids! I just had to shave all mine down AGAIN and re-treat them; I already treated the does but didn't get the kids the first time, so of course it came back. I've also noticed that here, babies seem to stop growing if the heat is extreme, like I get in July/August. They pick up again once it cools.