The Shadow on the Dial, by Anne Lindbergh (1987), is an example of time travel that teaches a modern kid how to be a Better Person. Two not especially likeable siblings, 12-year-old Dawn and her little brother Marcus, are dumped on their great-uncle, who lives in a retirement community in Florida, while their parents go off on a vacation. Their uncle doesn't really know what to do with them and the kids are bored. Dawn fantasizes about all the wonderful things she'll accomplish in music and dance, but doesn't do any practicing (this is clearly what needs to be changed about her), and their uncle expresses bitterness that he never learned to play the flute.
However, their visit soon turns much more interesting than they'd expected. Marcus has pilfered a coupon for One Heart's Desire, Deliverable on Demand, with the cryptic instruction--"just dial." And it turns out that fooling with their uncle's sundial counts, and it turns out that it's the heart's desire of someone else being offered, so...they travel back in time to all the moments when things went wrong for their uncle viz flute playing, from kid to adult, and by the end of it they've changed the past enough so that he had a happy career in the Boston Symphony and ended up married. And Dawn has learned that daydreams aren't enough to make a happy future for yourself, and so the reader goes off to practice their own musical instrument or whatever.
The time travel is actually rather nicely done, and is what makes the book readable, and even enjoyable. The visits to the past, and one to the future, are fun--interesting characters and situations. It's also interesting to see how the chain of events spins itself out. It's not one thing alone that kept the uncle from learning how to play the flute, but a whole sequence of attitudes and events, from his father's attitude that no son of his would do such a sissy thing, to a beautiful girl who tells him he looks stupid when he plays, to an addition almost missed because of walking a girl home (and a few more). And Dawn and Marcus are challenged each time to figure out a way to keep the impediment from having long term consequences, and rise to the occasions successfully and believably.
Huh. School Library Journal thought somewhat differently--"The manipulation of so many events to accomplish such astonishing changes is not convincing." It's one of those books where you have to just accept the premise that lots of changes will happen (because that's the point), and go with it. I don't think the SLJ reviewer had read much time travel. And indeed, back in the eighties there wasn't much to be read; out of the 200 or so time travel books for kids and teens I've reviewed here, there are maybe five from the eighties, 2 of which are 1980, and so don't count, and 2 of which are from outside the US. I have a vague plan to someday try correlate the quantity of time travel books being written with social and political trends. The Reagan years seem especially unconducive to time travel fiction--is there a connection????
Anyway. The Shadow on the Dial isn't particularly dated, and if the modern young reader feels slightly warmer to the two spoiled brats dumped on their poor uncle by thoughtless parents than I do, and is able to get past the cover illustration, they might well enjoy the time travel part.