> the point was to keep a puppy or two
Please, please please only keep ONE puppy yourselves. If you want to know why, google 'litter mate syndrome' and 'should I have one puppy or two?' and so on... And watch the many videos and articles that appear. Such as these:
https://youtu.be/LSEno_YxhMk https://youtu.be/jfZ5n-N1sJkhttp://www.whole-dog-journal.com/issues/13_1/features/Problems-Adopting-Two-Puppies-At-Once_16190-1.htmlhttp://blog.betternaturedogtraining.com/2013/07/18/littermate-syndrome/Secondly, I think it is very irresponsible to say BOTH that you don't think the mating was avoidable AND that you're not going to spay your dog. I've owned intact bitches for about 15 years now (no males), and I've never had an unwanted mating in all that time. I've never had even a slightly risky encounter.
All dogs should have a reliable recall, but if you're going to keep an intact bitch then that definitely needs to be in place. And harnesses should be secure and unable to be backed out of. There are harnesses with two straps which are impossible for a dog to back out of. Being in heat doesn't somehow give dogs incredible harness-escaping powers beyond the ordinary. They could equally escape to get to a person or dog on the other side of a road...
But you can't ethically go around saying that it was a completely unavoidable mating and also that you're not going to spay your dog. It's essentially inviting the same thing to happen again. Why would it not? You're not changing anything because you don't think it was avoidable and you're not spaying the dog. The latter I ordinarily completely agree with, BTW - owning an intact bitch doesn't equal breeding from her. But if you can't manage her to the point where you can prevent her breeding, then those are IMO the owners who SHOULD be spaying. And in your own words, you are one of these...
Train your dog. And/or keep your dog on a secure harness. And if you can't achieve either of those and aren't going to change anything, then please spay her.