All heroes need a bumbling sidekick. How else would the hero look awesome by comparison? You think Batman dresses Robin up like that because it helps with the crime fighting? Pshaw. Batman just wants to look cooler when standing next to the kid. While Darzee and Chuchundra aren't "sidekicks," both act as foils to Rikki-tikki because they provide counterpoint bumbling.
Darzee is a birdbrained, uh, bird who doesn't realize when it's the right time to do anything. He's stoked about Nag's death, without considering that Nagaina remains very much alive and probably very pissed. Chuchundra is a coward of a muskrat. He only comes out at night, and he fears talking to Rikki-tikki because the cobras might kill him by mistake.
The story heightens Rikki-tikki's positive characteristics by contrasting him with these two bumblers. Unlike Darzee, Rikki-tikki knows when it's the right time to act. Unlike Chuchundra, he remains brave in the face of danger. Although both characters help in their own way, their major contribution to the story is making Rikki-tikki look cooler.