I can thoroughly recommend Camp-du-Lety at Benodet. Right on the beach (clean and safe for young children). The site is spotless and the toilet/shower facilities are first class. Benodet is a beautiful area of Brittany and quite central for trips out.
The road from Roscoff to Quimper is dual carraigeway and first class. The only downside in approx. 80 miles there is only one fuel station so make sure you have plenty of fuel. Quimper to Benodet is approx 10 miles of normal road but very good for caravan towing. Our only disappointment was a visit to Brest - dirty town and a big let down.
The ferry from Plymouth to Roscoff takes about 6 hours but look upon it as part of the holiday. We travelled down the day before sailing in case there were hold-ups on the M5. We stayed overnight on the quay side at Plymouth. Basic toilet/washing facilities but at Roscoff the facilities at the port are brilliant.
Good luck and enjoy your holiday - wish I was going with you.