Young adult book I read in the late 1970s/early 80s about kids who find out a large tree is a time machine traveling in 50-year increments. Their parents went into it (I think?) and in the end they decide to go into it as well knowing when they come back everybody will be older.
This was an illustrated children’s book about a family of pigs whose parents left them with a terrible/villainous babysitter. They had to save each other. Does this ring any bells?
My 4th grade teacher in 1958-59 read us a chapter-a-day in a children’s book about a group of kids trying to solve a mystery. While trailing a bunch of crooks, the smell of the kids’ mosquito spray (quinine) gave them away. Calpurnia was the housekeeper.