I finally found my son who was lost in the shopping mall.
We finally located the little boy who went missing in our neighbourhood this afternoon.
この場合、次のような言い方ができますよ。
ーI finally found my son who was lost in the shopping mall.
「ショッピングモールで迷子になっていた息子をやっと見つけた」
ーWe finally located the little boy who went missing in our neighbourhood this afternoon.
「午後、近所で行方不明になっていた小さな男の子をやっと見つけた」
ご参考まで!