I saw them【 playing football in the playground】因为这里的地点状语是修饰宾语补足语的 而不是 saw

I saw them【 playing football on the playground】.playing football on the playground---整个这一段是现在分词短语作动词saw的宾语补足语（them为动词saw的宾语）.即词组---see sb.doing sth.on the playground-...