Whose idea was it that they travel to Oakland?

It's Pa who wants his three girls to go visit their mother Cecile in Oakland. The girls haven't seen their mother since the youngest sister was born seven years ago, and so Pa thinks it's high time his daughters knew Cecile. Big Ma's not very enthusiastic about the idea. For one thing, Cecile abandoned her kids when she walked out on Pa. Moreover, Big Ma's worried about Cecile's involvement with the Black Panthers. She's heard about them on the news, and thinks they're "bad and dangerous." The last thing she wants is for her granddaughters to get mixed-up with such unsavory characters.
But Pa's mind is made up; Delphine, Vonetta, and Fern are going to spend the summer of '68 in Oakland with their mother. The girls will indeed come to know their mother, and much else besides, on their incredible adventure, though not in quite the same way that Pa envisaged they would.