Kids pick up on words in a snap. One time, my toddler happened to be in my lap when I was stymied by a particularly tough level of Candy Crush, and I let out a frustrated "Well, sh*t." For the next two weeks, guess what he said every time he got frustrated? Yep, "sh*t." If he couldn't reach something on the counter: "Sh*t." If he couldn't snap two Legos together: "Sh*t." If I wouldn't allow him to have mustard for lunch: "Sh*t."
It only took me saying it once, albeit right in his ear, to embed it into his brain, because kids have an amazing capacity for language (whether we want them to or not). This is actually something we can take advantage of, though. Because little kids' vocabularies are developing at an astonishing rate, it's prime time to teach them all kinds of words — even beyond the basics — and set them up with an "extensive repertoire" of words to choose from rather than "a whole bunch." (See what I did there?)
A broad vocabulary will serve them well, not only throughout school but in adulthood too, and give them a wide variety of ways to verbally express themselves. It's like paint: why limit them to red, blue, and yellow when we could also — just as easily — add shades like cerulean, indigo, and crimson to their proverbial palettes?
Sometimes we underestimate the abilities of little people to learn big words, but you'd be surprised. They may have to work on the pronunciation a bit more, but at the very least, they'll know what it means. For example: when we're feeding our babies some sort of mushed veggies, we say, "Yummy!" — and we also say, "Mmm!" to convey the same thing. We say these things because they're simple (the words, I mean, not the babies). But while we're trying to convince them just how "yummy" those pureed peas are, we can also say, "Delicious!" and "Scrumptious!" and "A gastronomic delight!"
OK, so I'm kidding about that last one, but you're picking up what I'm putting down: even if they can't say it right away, they'll understand that there are several ways to describe something.
A firetruck can be "big," but it can also be "gigantic," "enormous," or "colossal," among other things. And teaching our kids all the ways to explain it is as easy as using a different word every time we point something out: "Wow, look at the size of this apple! It's enormous!" or "Whoa, that's a gigantic tower of blocks you've built there!" There are teachable moments all around — like telling them they've done a fantastic job instead of a good one, asking them if they can leap all the way from here to there instead of jump, praising how intelligent they're getting instead of how smart.
If they look truly confused, you can pair the unfamiliar word with a basic explanation — like the time I told one of my kids not to touch the "decorative" items and then clarified that decorative means "pretty to look at but not to put our fingers on." But for the most part, kids are only limited by our tendency to dumb things down for them when they don't really need it. You'll be amazed at their capacity to build an impressive vocabulary, even from a very young age; in fact, the younger you start, the better. They'll soak it up like the little sponges that they are — and then the words they're repeating will make you proud, not mortified.
. . . Just don't play any Candy Crush while they're around.