The best term remains understatement:
a statement that represents something as smaller or less intense, or less important than it really is.
Hence we have the Monty Python sketch that includes the Army officer who has just lost his leg. When asked how he feels, he looks down at his bloody stump and responds, "Stings a bit."
So, "it's a little warm", when the temperature is 115 F, is an understatement of the true temperature.
If you choose to go even further and interpret "It's a little warm" as a negative statement such as "it's not really hot", the usage might be described as litotes.
understatement, especially that in which an affirmative is expressed by the negative of its contrary, as in “not bad at all.”
This definition fits your example but Cambridge emphasises that litotes is usually (although not always - see the Dictionary.com definition) used with negatives intended as positives.
the use of a negative statement in order to emphasize a positive meaning, for example "a not inconsiderable amount of money (= a considerable amount of money)"