the only junior co-ed I know anything about in Dulwich is Alleyn's - I had a look round for dd but decided against as it's just a little too far for us, but it looked good and I know plenty of kids at the senior school
it is competitive to get into, but as far as I remember the 4+ assessment was play-based and the 7+ wasn't too formal
once they're in, they're pretty much guaranteed transfer to the senior school at 11, which is a big bonus as getting into Alleyn's at 11+ and 13+ is a bit of a lottery
however, there are a lot of very rich kids there, and I'm not sure if it would 'round and ground'
It is much less socially and ethnically mixed than places like JAGS and DC and certainly the Croydon schools
ds knows a couple of kids at DC junior and loads of boys at DCPS - DCPS seems to be much more academic and generally a bit more 'full-on', which may or may not be a plus for you ...
ds1 goes to an all-boys school which is perfect for him - he's bright, sporty and extrovert. However, shy rather geeky ds2 will be going to a mixed school as I think the atmosphere at ds1's school is a bit too testosterone-fuelled for ds2 ...