It's a Ghibli adaptation of Mary Norton's 1952 novel, The Borrowers. (There have been plenty of others, incidentally. The BBC did one the following year, starring Christopher Eccleston.) This wasn't directed by Miyazaki, by the way, although he oversaw the production development and co-wrote the script.
It's a nice film. It's not a major work and there are features of the original story that make its ending a slight downer, but it's beautifully animated (of course) and pleasant to watch.
It's about Borrowers, as they generously call themselves. (They "borrow" our food, electricity, etc. A more objective word might be the Thieves.) However these pilferings are harmless, since they're smaller than mice and could be eaten by a passing cat. Our heroes are a family of three: Arietty (nearly fourteen), Pod (her tough but nice father) and Homily (a timid mother who looks more like an elderly aunt). They live under the floor of a house in the country, believing incorrectly that they've never been seen by humans and maintaining strict rules of self-preservation. Never be observed. Never go near a human.
However there's an ill boy (Sho) in the house who's more observant than anyone thinks. The relationship between Arietty and Sho would end up looking romantic if she weren't the size of a toothpick and if he weren't about to have heart surgery with low chances of success.
The animation's a good reason to watch the film. It's very pretty, while it's interesting to see Ghibli animating daily life at three inches high. (I'm reminded of Antz and A Bug's Story.) Water doesn't flow or pour as it does for you and me, for instance.
That said, though, the film's setting is a deliberate fudge of countries and decades. The word is "timeless". They've moved it to 2010 and to Japan, as can be demonstrated trivially by the shiso and tanuki in the garden, but the house's furniture and architectural style is mostly British (although the roof tiles are Japanese) and a lot of the decor and kitchen appliances are very 1950s. (If you're watching on DVD, though, don't judge by the note that's apparently been written in English. I'm pretty sure that's a big, clever subtitle.) I wasted plenty of effort trying to work out where we were, but I think the best way to approach this film is just to roll with it.
The plot's fairly lightweight. Will the Borrowers be discovered? Pod and Homily think that would be the end of the world, but we know it would be harmless if we're talking about Sho and his grandmother. With Haru, unfortunately, that's less true. She's the film's villain, although quite a low-key one. She has a few amusing facial expressions, though. She made Natsuki laugh. (She also gets uglier as she gets more threatening, with her face ending up either simian or frog-like.)
What we're really watching, to be honest, is Arrietty and Sho. We like them both. We'd like them to get together, but that's not possible. The ending doesn't do any of the things I'd been wanting it to, although it's still pleasant and everyone's doing the right thing. It's not depressing or anything. (The 2012 Disney dub apparently adds a final monologue to reassure us that Sho's operation was successful, but there's also a 2011 Studio Canal dub with UK actors that's more faithful. Both have their fans.)
It's a nice film. Children should enjoy it. There's absolutely nothing wrong with it.