She tried to protect her child at the cost of her own life.

Literal

She [topic-は] life [object-を] risking child [object-を] protect tried.

命を懸けて (いのちをかけて, 'staking one's life') uses 懸ける (かける, 'to stake, wager'). 守ろうとした uses the volitional + とする pattern for attempted action. A sentence about maternal sacrifice.