Hello! I'll be traveling to England for the first time ever in August, with my family that includes 5 adults and 2 very young children. Trying to decide on a location that we'll all enjoy, with enough things for the adults to do yet a place that works well for a 3- and 5-year old. My ideas are all over the place, from Cambridge to a larger seaside town in the south. We aren't looking for any of the typical touristy attractions, just some great sites, walks, and new experiences. Would appreciate any help!
I know this isn't what you want to hear, but London is great for young children. We took our daughter when she was 4 and the first photo she ever took was of a Beefeater at the Tower. There's a great Natural History Museum and Kensington Gardens. Maybe stay in Richmond or Hampstead(that's where we stayed because my husband was working at a hospital in that area) and take trips into central London. I've been to Cambridge twice and love it, but don't think there's much there for young children.
England has dozens, maybe even hundreds of such locations depending on your particular interests, but I would suggest not overthinking this for a first visit. London. There's good reason it's one of the most popular cities in the world. It's got a lifetime's worth of things that appeal to both kids and adults, including (for the kids) great parks to let off steam, boat ride on the Thames, etc. You want "great sites" but not "typical touristy." OK, but if something is a great site, it tends to attract other tourists, and I don't think that's sufficient reason alone to avoid it -- examples being the Tower of London, British Museum, Westminster Abbey. Do you really want to go to England and avoid all of these? What I would describe as "typical touristy" but NOT a "great site" are those places that are not significant in their own right and therefore appeal to tourists, such as the Tower, but those that have been established recently ONLY for tourists, such as the Harry Potter tour or the "London eye" ferris wheel. These I agree are worth avoiding, although there are many who love Harry Potter and wouldn't miss that tour. You don't mention how long your trip is, which could make a difference, since if it's longer than a week you likely could do two locations such as both London and a smaller place such as Cambridge or Bath.
You mention the south coast. Will you have a car? You can get around by bus and train but a car will make things easier. Also, sadly, most of the south coast beaches closest to London are shingle rather than sand.