Late July > early August is the peak holiday season and Dorset is a very popular area for the Brits to take vacations. If you can do the trip in June or the first half of July, it will be less busy as the schools are still open. You are choosing the very start of the school summer holidays.
I would not advise getting off a trans-Atlantic flight and driving to Dorset on the same day. If you want to go to Dorchester from Heathrow - take a bus to Woking >https://www.railair.com/timetables/RA2 and then a train to Dorchester South station. (You might want to check in to the Premier Inn - which is right outside Dorchester South station). It might work out cheaper for you to buy return tickets than one way tickets twice.
If you want to rent a car, I see that Enterprise have a depot in Dorchester - on the north side of town, (Looks like you will need a car to get to the village - which is not far east of Dorchester). https://discoverdorchester.co.uk/
You have never travelled abroad. Trust me, there is loads to see and do in southern England so I would forget about Paris. Besides, why would you go to Paris when it sounds like you have never been to London?
If staying in the Premier Inn, it is quite easy to go to the Dorchester South station and take a train down to the seaside town of Weymouth. The girls could easily do this on their own - should it he desired. Bring swimwear as the sandy beach is safe and lovely. You could also take a short walk to Dorchester West station and take a train north to visit the historic city of Bath. (Going by train would be easier than driving to Bath). Trains >https://www.nationalrail.co.uk
A few miles SE of Dorchester is Lulworth Cove and Durdle Dor (spectacular coast of bays and cliffs).
The village of Cerne Abbas - just N of Dorchester has the famous giant on a hillside - best seen from west side of valley. (Many excited women present when I visited). Further up the valley is Minterne Gardens and the town of Sherbourne is also worth seeing -castle is on S side of town.
You might also be interested in this bus service:> https://wdlh.co.uk/jurassic-coaster-bus/
https://www.jurassiccoast.org/explore/towns-and-villages/lyme-regis