Most people (in pre-pandemic times) would fly from Bristol to Edinburgh/Glasgow to get to Dundee. The flight is generally cheaper than the train and a lot quicker. The drive takes 8-10 hours in a car, even longer by National Express/Megabus but they're not currently running except within Scotland. So either she travelled up there by train, plane or car. Last I heard everyone was being questioned at Bristol Airport their reason for travel during lockdown but of course children are still allowed to travel to see parents so it's still possible that they travelled by plane and that that was the reason given, but in that case the police would have known straight away when they were reported missing that they were in Scotland. I also find it strange that no-one reported them missing sooner, because if she was living in accommodation for people escaping DV like it has been suggested, then you would think that someone would be checking in on her regularly, like a family support worker since she had kids. She was last seen in the middle of the Feb half-term so that makes me think perhaps they travelled up to Scotland during the half-term which would have been less suspicious if people saw a woman with children travelling during the half-term. I also wonder if the 7 year old's school had managed to make any contact in terms of home-learning. The school may have been the ones to report them missing?
(All just speculation/my own opinion etc.)