。
She was carrying the baby on her back.
Literal
She [topic-は] baby [object-を] was-carrying-on-back.
おんぶする is a colloquial verb meaning 'to carry on one's back,' specifically used for carrying children. It's more casual and child-focused than 背負う (せおう), which applies to carrying anything on one's back. おんぶ is baby/child vocabulary — the kind of word children learn early. The ~ていた marks the ongoing past state.