You might want to check with the manufacturer or a veterinarian but I'm reasonably sure you're cats would be fine. DE doesn't kill insects chemically so it's not a toxin. It kills them mechanically by cutting into shells and skin, causing them to dehydrate. They act like little glass shards on a microscopic level, getting into tiny crevices in their joints and exoskeleton. Like humans, your cat's skin can easily survive microscopic abrasions without harm. In fact, the ground is probably rougher on its skin than powderized DE. As for licking it, because it's not poisonous, nothing will happen. In fact, any dry cat food would be rougher than DE on its tongue and gastrointestinal system. Furthermore, this is Food Grade which means farmers mix it into their grains to kill pests in storage. Plus, DE loses its killing power once wet because it clumps together and the sharp edges become dull. In other words, your cat's saliva completely disables it, so that even insects won't be affected.
The biggest issue is inhalation. Humans are told to wear a mask when applying large amount of it. However, once the DE goes on a surface, i.e. wall or floor, it should cling and no longer be airborne. Remember to only apply a light dusting and not mounds of powder (buy an insecticide duster). As such the powder should not be floating around to be inhaled, and only get on the insects through rubbing on the wall and floor. So unless your cat makes a real effort to suck it in, i.e. like humans sniffing cocaine, there should be little chance of your cat inhaling significant amounts. Keep in mind that there is nothing especially attractive about DE to cats. And DE is generally applied to cracks, crevices, edges, behind furniture, etc -- not areas cats especially like.