If you search through some of the archives (particularly around the BBC relocating) there are some very long threads with all sorts of tips about areas etc.
Personally, I would start with Trafford (because educationally, its the best place in the country to live, nevermind just in Manchester - and thats according to The Times) so areas such as Altrincham, Hale, Bowdon, Sale, Brooklands. I don't know where you live in London but there was a thread quite recently which compared some of the areas around Manchester to London areas. Schools are over subscribed so you'd have to do your homework, particularly for your 8 yr old.
But there are other boroughs in and around Manchester that are also great, schools are still good, access to green, open spaces. We were in Didsbury yesterday which was lovely, shortage of school places there too, some of the areas around Stockport are nice too.
I moved out of London before I had children so can't comment on how easy it will be with children, but one of our neighbours moved when the BBC relocated to Salford Quays with a very young family and they say they were wary to start with (everyone seems to think life stops north of the Watford Gap and everything is as cheap as chips - neither is true!) but they love it and say its paid dividends for them.