This is the 13512th most frequent Russian word.
нарядные
Smart, elegant, or dressed-up.
нарядные in this sentence is used as an adjective describing the guests' elegant or festive clothing.
На вечеринке все гости были нарядные.
At the party, all the guests were dressed elegantly.
нарядные here emphasizes that the children are dressed nicely for the occasion, suitable and formal.
Дети нарядные пришли на утренник в школе.
The children came dressed up to the morning event at school.
нарядные in this context refers to the dresses being attractive and suitable for the festive event.