。
The children couldn't see the daisies dancing in the moonlight.
Literal
Children [topic-は] moon-night [at-に] dance daisies [obj-を] see [nom-こと][subj-が] not-able-to-do-[polite past-ませんでした].
A literary, fairy-tale-like sentence. 月夜 ('moonlit night') is a poetic compound. 踊るひなぎく is a relative clause — 'dancing daisies' — personifying the flowers with the verb 踊る. ひなぎく ('daisy') is usually written in hiragana. The potential ~ことができる paired with the polite past negative gives 'were not able to (see).'