We loved Keranmeriet when our DC were 4 and 6. The gites were the best furnished of any we found. The English owners were helpful but not intrusive.
Sadly the weather was pretty drizzly when we went but there were so many other children staying so they all got together and entertained themselves.
Pont Aven is a lovely village as is Concarneau which had a deserted soft play centre. There is a wonderful beach about 20 mins away which has a stream running into the sea which the kids loved playing in.
Quimper is a beautiful medieval city but also has lots of funfair rides for the kids.
We also went to a great amusement park. Health and safety was zero (zip wires over a lake with no attendants and a rickety toboggan run into a wall of tires) but we had a blast.
www.kerlividic.com - we've had two wonderful summer holidays here and thoroughly recommend it. There are just 3 gites with a lovely shared pool, beautiful, quiet location in the heart of rural Brittany - very child friendly, clean, well presented accommodation with everything you could need, and really welcoming owners. I searched long and hard and its great value for money! Happy to share more info if you like!