Interesting question. I suppose the question would be what each family member does as a runner. Based on what you said, it looks like magic might "run in the family" so to speak. There's no proof that magical ability is hereditary, but it's always a good story hook.
If I was developing a runner with that kind of background, I guess I'd start by figuring out when they likely stopped running regularly. With kids in the picture, it's hard as nails to run regularly so I would imagine they likely have about 10 years of running experience before getting out of the game. So I'd probably build a regular character and then add approximately 10 years of Karma to them. I like to say that a campaign takes approximately 3 - 6 months to complete, and most of the characters in my campaigns gain about 30 Karma per campaign. I don't make anyone use downtime for training, since I figure that into the length of the campaign already.
What you could do is look at what you make initially for the characters, look at what you want them to become (numerically) and then calculate the training time to get to that desired numeric value. That would help you get an idea of how long they would have to be runners to get to that point.