Building Blocks, by Cynthia Voigt (1984).
Brann's parents are at odds with each other. His dad has just inherited a run-down farm in Pennsylvania, and wants to move out to it, his mom wants to start law school in New York. It is the 1970s, after all--and a woman should be able to make something of her own life....And Brann feels battered by their fight about it, so he retreats to the basement, and begins to build with the blocks that were his father's back when he was a kid.
And Brann travels through time, and arrives back in the Great Depression, in his father's house, and there is his own father, Kevin, a kid a little younger than Brann. Spending the day with Kevin, and Kevin's family (dictatorial, scary father, worn-down mother, and bratty siblings) gives Brann a new respect for his father's hard-won, quiet courage, and when he comes back to his own time, he's able to come up with a solution that keeps his own family together.
It's a simple story, and I remember when I first read it, back many years ago, being somewhat disappointed that more magical-ness doesn't happen. The things Kevin and Brann do together are prosaic--they eat breakfast, look after the bratty younger siblings, explore a cave, and trespass to go swimming. Brann does not run into any time-traveller difficulties; he is accepted as a contemporary kid.
But reading it a second time, I have more appreciation for the quite character study that is the book's strength. Poor Kevin really does have a thin time of it, and Brann comes home from his experience a wiser, more understanding person, which puts this one squarely in my "time travel as opportunity for emotional growth" category. Unfortunately I think it edges into "book as an opportunity to push emotional growth on reader" territory, and though it is a just fine book, I don't think I ever need to re-read it again (having grown as much as I'm going to from it).
(Is there an online archive of SLJ best books of past years? I'd love to browse through it, and a quick search came up empty).