We Did It Anyway by Carla Salmon

Treasure. Smugglers with guns. No way off the island.

Luna would do anything for her best friend, Milly, but why did she agree to spend a week roughing it on an island?

Alex, loves the idea of camping on a beach with his friends Otto and Jasper. But their best-laid plans spiral out of control when they discover on a fortune in gold bars, and have to flee from armed smugglers who want it back.

Alarmed when the boys miss their designated rendezvous, Luna and Milly have to figure out what's happened.

Can they save their friends?

Before it's too late.