The author could have used a comma, but they used a full stop to give you an understanding of the mood of the character. It is as if the character is considering what to do next and thinks about it carefully, before deciding. Also, it suggests that it is a bit of a strange thing to do.

Yes, it would be perfectly correct, and slightly more usual, to join the sentences with a comma (and not capitalize "so"): "The children were at school, my husband was at work and the house was quiet, so I decided to make some meat pies." It is a matter of style. A sentence corresponds to a single thought. If two ideas are closely related and flow into each other so that they feel like one thought, we write one sentence. If they feel like separate thoughts, we write two sentences. When my mother first explained punctuation to me, she explained it as a way of indicating pauses in speech. A comma shows a short pause. A period or "full stop" shows a long pause. The phrase "full stop" suggests that you actually stop speaking and take a breath. I understood her perfectly, because I already knew spoken English. Punctuation in writing and pauses in speech are used to show grammatical structure, too. She didn't talk about grammar because I had learned the grammar of short and long pauses intuitively. The way the author has written it, in two sentences, suggests a thought process like this: "The children were at school, my husband was at work and the house was quiet. [Hmmm, I have a chance to do something I don't usually get a chance to do. What should I do? Read a book? Watch television? Aha! There's that meat pie recipe I wanted to try.] So, I'll make some meat pies." The material in brackets flashes through her mind during the little pause between the sentence.