。
She has cherished irreplaceable memories from her childhood.
Literal
She [topic-は] childhood [from-から] irreplaceable memories [object-を] is-cherishing.
かけがえのない is a beautifully fixed adjectival phrase meaning 'irreplaceable, one-of-a-kind' — built from かけがえ ('substitute, replacement,' an old noun) plus the negative ない. It pairs especially with 命 ('life'), 思い出 ('memories'), 友人 ('friends'), and other things you can't get back if lost. 大切にする ('treasure, value, hold dear') uses する as a state verb here in the present-progressive ~ている form, marking the ongoing, lifelong nature of the cherishing.