@mohkamfer,
mohkamfer wrote:
The children were covered in sand, they (were playing - have been playing - played - had been playing) on the beach.
If they're still playing on the beach, use simple past: were.
If they've finished playing and are now going home or something, use the past perfect: had been. The present perfect, "have been," doesn't work here because it'd be a verb tense shift. The simple past, "played," doesn't work because we use the perfect tenses to indicate that a past action, though complete, still has some relevance to the later (or present or future) state, ie the kids being covered in sand.