I recently had a debate with a friend about whether "littlest" was a word. I took the stance that it was not. I find now that basically every time I make such a claim these days someone can hold up a phone with some web dictionary "proving me wrong".
My question is: When is it appropriate to use this word as opposed to smallest or some similar option?
Littlest is not 'standard', but perfectly acceptable in conversation. What you should be concerned about is that it has overtones of cuteness and childish language - consider The Littlest Mermaid, or Google the word and see the contexts in which it is used.
I think you will usually want to avoid it. However, the same can be said of its 'standard' alternative, least, which is strongly marked for formality. Except in fixed phrases like the least of your concerns or I haven't the least idea, where least is obligatory, practically nobody uses least colloquially to mean the superlative of little (or less for its comparative).
Smallest is safest. Smallest will never make you sound like either a kindergarten teacher or a college English professor emeritus.
You can safely use littlest when you're talking about Lilliputians (Jonathan Swift's creations in Gulliver's Travels), and the "Little People" (leprechauns) of Irish folklore, and small-minded bluenoses (American Puritans) with with hearts and worldviews as big as the pea that disturbed the fairy-tale princess's sleep.
Little has negative connotations that make it worth using to cut someone down: "You are a little man despite your Gargantuan stature. The littlest man I've ever had the misfortune to meet, in fact."
You can also use it if you're my friend from Texas, who keeps telling me about all the "little tiny towns" he's been to in his home state. One of those towns is surely the "littlest tiny town in Texas". And since you're from Texas too, you probably know which towns he's talking about.
"Littlest" is one of those words that was not a proper word when I went to grammar school in the late 1950s. It was considered incorrect to use it, and "smallest" was the correct option. Of course, that was also a time when the 't' in the word "often" was silent and not pronounced, and the pronouns " who" and " whom" we're not used interchangeably as they are today.
