We did this last week with a 3.5 year old and a 5 month old. Left at 8pm with kids in pj's, they slept as planned! Took 3.5 hours to exeter where we stayed overnight in a travelodge (mainly just as we couldn't check in till 4pm in Newquay the following day). The next morning had leisurely breakfast then 1hr 20 to Newquay. HOWEVER on the way home we left at 11am, big mistake on a Friday, took us 10.5 hours to get home!! Was the journey from hell, baby was so unhappy and screaming, we stopped when we could but when you're on a motorway and it's 20 miles to the nearest services and he's screaming blue murder it's heart breaking! I advise leaving either v early or late, the traffic getting out of Cornwall and on the m4 is really slow at that peak time. Have a great holiday, we love it down there Xx