Taking Care of Terrific — Lois Lowry

Fourteen-year-old Enid Crowley can't stand her name. When she takes a summer job babysitting young Joshua W. Cameron IV, Enid decides it's time for a change: she calls herself Cynthia, and Joshua becomes Tom Terrific. Every day they're off to Boston's Public Garden, where Enid hopes to meet people and find excitement.

It doesn't take long before Enid and her new park friends -- Hawk, the saxophone player, and the old bag lady -- are involved in the wildest adventure the park has ever seen. Their project is top secret. Nothing can go wrong. Or can it?