The Boxcar Children spend most of the novel trying to evade their grandfather. In fact, the dude's the reason they ran away in the first place—they've never met him, but he has a certain rep. "We have a grandfather in Greenfield, but we don't like him" (1.17), Benny explains. "He is our father's father, and he didn't like our mother," Henry adds. "So we don't think he would like us. We are afraid he would be mean to us" (1.21). Who's the big bad wolf? As far as the Alden kids are concerned, it's clearly their grandpa.
As it turns out, though, their grandfather is perfectly nice—or, at least, he's nice to the Boxcar Children. (Did he really hate their mom? We'll never know.) His name is Mr. James Henry Alden, he's filthy rich, and he's so desperate to find his family that he's put a "LOST CHILDREN" ad in the newspaper.
His patience and efforts are ultimately rewarded. He finally gets to reunite with the Alden kids at the end of the book, when Dr. Moore—who has long since figured out the Boxcar Children's identities—deigns to tell Mr. A. what's up. Thanks, Doc.
From the get-go, Mr. A. is very generous and considerate of his grandchildren's feelings. For one thing, he introduces himself as a rando so they won't be afraid of him; for another, he redecorates a wing of his house so they'll feel excited about living with him. Then, finally, after they move in, he hauls their dirty old boxcar into one of his fancy gardens as a special surprise.
Who knows how Mr. Alden was as a father-in-law; all we can say is that as a grandfather, he seems A-OK.