They are having a picnic in the park at the moment.
句意:此刻他们正在公园里野餐。
陈述句语序:主语(They)+谓语(are having)+宾语(a picnic)+地点状语(in the park)+时间状语(at the moment)。由时间状语at the moment(现在)可知该句为现在进行时,故谓语动词have(有)需变成现在分词having。主语为第三人称复数,故助动词需使用are。故填:They are having a picnic in the park at the moment.