Question

It is raining.The children could not play.(Join into complex sentence)

Answer 1

The children could not play due to raining.

Answer 2

The children could not go out to play because it was raining.

You can join two simple sentences to make a compound sentence.

In this case, you have to join the two independent clauses, and you can use comma between independent clauses to join the sentences.

But that can be applicable for the coordinating conjunction only.

Normally, you can join the sentence by using noun and phrases.