You either must be brave/mad to attempt such a long tedious journey with children so young or they must be excellent travellers! Three nights only means 2 days there as it will take the best part of a day in each direction. Porthleven is a fair way west in Cornwall and driving from north London is likely to take between 6 and 8 hours with only a brief bathroom stop. I got unlucky once in traffic and Plymouth to London took me over 10 hours and this is a shorter journey. Don’t attempt it at the weekend in the summer.
Do you really have an interest in Stonehenge? It takes several hours to see it (it wouldn’t be on my top 100 places to visit list in England). But it’s not my trip!
As you have already considered, there is no easy option. For a party of your size, you are going to need either 2 cars or one large people carrier. The latter will be more difficult to source and you may have to go to Heathrow airport to find so you have the hassle of all getting out to Heathrow. I don’t know where you will be heading in Cornwall, but there are many narrow lanes there, so a smaller vehicle would be easier to manage.
Petrol/diesel is currently around £1.30 per litre (£5.85 per gallon) and seems to be increasing at a penny a litre every week as more people have started driving again after lockdown. It will be significantly more expensive on the motorway and in rural parts. Pre Covid, fuel was around £1.50 a litre.
Children under 4 travel free on trains. Buying seats 10 weeks out and travelling after 10am is the cheapest option. Hiring a large people carrier may not be possible in Exeter.
There isn’t one solution for you - it just depends on what your party wants/ is prepared to put up with. The journey is long whichever way you try to get there. I don’t have young children and I am U.K. based so know the roads and drive a manual and I would have to think twice about undertaking such a long journey for such a short time there.