The reversal of the normal and "natural" order continues. Once again, the theme is the contrast between the expected danger, and the complete peace. The image of the lion eating straw is much more of a statement of the power of the symbolic peace than it is a scientific alteration of diet.
Even innocent and incautious children will be safe from the less intelligent, though still deadly animals. Both the asp and cockatrice are serpents. The NIV uses cobra and viper respectively as the translation. The image is still of the young child able to safely place his hand in the den of poisonous serpents.