Though it wasn't much, she gave the boy all the coins in her purse.
Literal
She [topic-は] were-few [but-が] wallet [possessive-の] inside [in-に] existing all [possessive-の] coins [object-を] boy [to-に] gave.
The opening 少なかったが ('they were few, but') sets up a concession before the generous act. 財布の中にあるすべての硬貨 is a nested modifier: 'all the coins that exist inside the wallet.' 硬貨 (こうか, 'coins') is the formal word, contrasting with the casual コイン. あげた ('gave') is the plain giving verb directed at the boy.