今天下午我和爷爷一起喂鸡了

I fed the chickens with my grandfather this afternoon.

My grandfather and I fed the chickens this afternoon.



以上两句皆可.
英语里通常用"this afternoon"而不时"today afternoon"来表达"今天下午"的意思,因为"this afternoon"是"这个下午"的意思,已能表明是喂鸡是你今天下午做的.