Ivy and Bean Make the Rules by Annie Barrows (author) and Sophie Blackall (author) (US edition)

Bean’s older sister, Nancy, is going to Girl Power 4-Ever Camp, where she will do Crafts and Music and First Aid and other secret things that Bean will never know about. Bean doesn’t care. She doesn’t want to go to camp. She wouldn’t go even if they begged her. So there. But wait a second! Bean and Ivy can make their own, even better, kids-only camp. Camp Flaming Arrow: where all the rules of summer camp are broken and the girls can wave goodbye to BORING!