I don't know Brenchley but have heard very good things about Brenchley and Matfield Primary School. Matfield is a very pretty village with pubs and a duckpond - close to Paddock Wood too which is handy if you need to use the trains. Mascall's Secondary School (Comp, not Grammar) is on the Matfield side of Paddock Wood - I know people with kids there and they seem very happy with it. Not sure if Brenchley's in catchment for Cranbrook Grammar?
And to answer your questions about Cranbrook - no I don't have any concerns about the school. One of their areas for development (which was also picked up by their recent Ofsted inspection) was to try to challenge and stretch the most able pupils further. The intake is very mixed in terms of socio- economic background (like the town itself) which can present challenges but is also one of the things I like about it - we're a diverse bunch and the school reflects this!
There's a very active and friendly PTA who would be thrilled to have a new recruit if you have time to get involved.
There are occasionally adults only nights - Quiz Nights, Disco, etc - which are a good way to get to know other parents, have a laugh and raise some funds for the PTA. Have I mentioned the PTA yet? Come join us ...
Cranbrook has a good choice of nurseries / pre-schools. My lot went to Juniors link up at Hartley Dyke near the farm shop 3 mins drive away. I chose them because their opening hours suited me as a working mum. And they stay open during school hols. But there's also Woodpeckers and Rainbows in the middle of town and know people who've been very happy with them.
If you're going to be sahm or only part time, there's the children's centre (right next to primary school) that does various activities, rhyme time at the library, messy church once a month and a toddler group at church too. Sweet little playground in between school and church. There's also this place which is apparently great, although I've never got round to taking any of mine for their craft / baking workshops. You won't be short of things to do with your youngest one!
My MIL's house on the High Street has just gone on the market (though not on rightmove yet) - am now nosily wondering if you're one of her viewers! 