Otter, chestnut and chinchilla are all born looking like that. Give it a few days to grow more fur, and then we can tell better which it is. Right now my gut instinct is with otter, though
Would they be Rex's? I have some Standard and Mini Rex that had castor babies start out as the same color. But like SableSteel said, give it some more time to see what you have. It's hard to wait but afterward it's worth it!