With Kristy's Great Idea, Ann M. Martin sets up the premise of The Baby-sitters Club, a series that spanned two decades, produced eleven different spin-offs and sub-series, and spawned a 13-episode television show and a movie. There's a lot ofBSC stuff, is what I'm saying; and if you're looking to get into it, here's where you should (probably) start.
In our first BSC installment, the club itself is formed. The Baby-sitters Club is Kristy's titular "great idea"; when her mother, Elizabeth, can't get Kristy or her older brothers, Sam and Charlie, to baby-sit for their younger brother, David Michael, she is forced to call all over Stoneybrook looking for a sitter. So Kristy gets together with four of her friends to form a club through which a a parent reach four potential sitters by calling a single number.
And that's how our club starts out. We begin with only these four girls--Kristy, Mary Anne Spier, Claudia Kishi, and Stacey McGill--and over the course of the next 350+ books, their lives (and an apparently endless loop of 8th grade school years) unfold. Characters form and sever relationships, characters move into and out of Stoneybrook, and some characters even die. But for now, we're starting with just these four girls and their initial issues: Kristy's divorced mother's relationship, Mary Anne's father's strictness, the possibility of Claudia growing up faster than and away from her friends, and Stacey's mysterious diet.
Kristy's Great Idea isn't one of the best books in the world, I'll give you that. But it's the starting point of a series that proved to be rather addictive, in and outside of its target audience. The books were a staple of 80's/90's "little girl's" literature, and though there are definitely scattered references that date the series, I would still recommend it to modern little girls. (And modern little boys. And modern not-so-little girls. And modern not-so-little boys. You get the picture.)
MG fans of all ages should check out The Baby-sitters Club at least once in their life; if you think the BSCbooks might be a little too much for your child(ren) to swallow at their age--whatever that age may be--the Baby-sitters Little Sister series or The Kids in Ms. Colman's Class spin-offs might be a better starting point. Alternatively, the California Diaries spin-off was directed toward slightly older readers. And for the mystery lovers out there, there's always the BSC Mysteries and the BSC Super Mysteries.
All in all, there's a lot of BSC stuff out there to try, and if you can get your hands on it, I'd always suggest giving it a go. It's not the most fascinating literature in the world, but it's charming at the very least.