To be honest, depending how old your children are and what needs they have, I wouldn't stop at all if you can avoid it.
We have done a similar run a few times (we're very near Bath), and it's been very quick. It's nearly always a really good run and we've never needed a stop-off, so stopping would only prolong the journey.
Unless you just fancy meandering down in your own time and having a stop en-route of course!