If you are in Broadheath or Timperley you will be fine for the Grammars.... but only if your children all pass for one of them, it is the alternative schools that you have to be more aware of if they don't pass.( It is a real possibility that bright children do not pass for grammars in Trafford due to the competition from both catchment and non catchment children ( well over a 1000 children sit the exam for each school)
If you would want Wellington school for example as your ' back up plan', you need to check on it's catchment rather than grammars which are very large, whereas wellington's is often not more than 1 than 1 mile away!
I am saying this from experience of the system which I really didn't consider when we moved to Trafford from Stockport for the schools, if I had my time again I would proabably consider staying in Stockport where schools are good/non-selective and houses cheaper:
We have 1 in girls grammar (annoyed she is in an all girls selective system now she's a teen, wishes she went to mixed/non-selective !) and 1 in (luckily) a good high school ( didn't pass and was devastated at the time, but doing well now and happy)
Choosing 6th forms is also more difficult than non selective areas as the local choice is limited to very formal school 6th forms ( not as 'grown up' as colleges elsewhere, strict dress rules etc which 16 year olds are not too keen on!),local college which is mostly geared towards those who didn't get to Grammar or travelling into Manchester colleges .