Charmouth is a really nice place as is most of that coastline. However, you might not find the beaches along that coast ideal for young children as they are very pebbly and the sea is quite dangerous as it shelves. You would find a lot of the beaches like that from Sidmouth round to West Bexington.
So if you want to stay in one place and go to the same beach every day (or as often as the weather would allow) Charmouth is not ideal. However, if you want to stay in an area and go touring around it is pretty good for that as the beaches and the area are good for exploring. Lyme Regis has a small but safe beach and Weymouth has a vast sandy beach and is it safe to go a long way out to sea although that is a much more touristy resort.
For more family-friendly beaches you might like to look at South Devon or North Devon - Woolacombe / Croyde Bay / Saunton sands are.