He was the only little kid at the picnic.这里为什么was?

He was the only little kid at the picnic.这里为什么was?
这难不成翻译成他过去是个小孩?到底是怎么用的高手指教
PS：在这里,我有必要声明一下,上次我提这个问题居然有人把picnic理解成“照片”意思,我晕
picnic 名词 n.1.(自带食物的)郊游,野餐;(个人自带食品的)聚餐[C]
不及物动词 vi.
1.去野餐,参加野餐
We'll go picnicking in the woods.
我们将去树林里野餐.
2.在户外用餐