You will love Glynn Barton Cottages. We've been twice and are going back in June. Can't wait!
I just thought that I would share (for what it's worth) that we had the same plan the first time we visited. Admittedly, we only had one 18 month old back then, but we set off at toddler bedtime and drove all the way down hoping that he would nod off and give us a peaceful journey. No luck! He was just going to sleep as we arrived at Bodmin Premier Inn!
The other thing to bear in mind, is that you can't check in until 4pm (they are quiet strict) so we had a rather long day trying to manage without all our stuff which was packed in the boot. (I'm sure you'll be better prepared than me!)
Last time we drove down, stopping for a leisurely lunch at a nice Alastair Sawday pub and generally taking our time and it worked well.
Anyway, have a super time and pm if you want to ask anything!