The problem is that interested (perhaps overly interested, pushy) parents want the best for their children and want guarantees of a great school and don't mind what consequence achieving that might have for other children.
Many parents want to have the option of a Grammar for their child and hope their child will be clever enough to get into one (and don't concern themselves about the children who then 'fail' and have to go to a school without the top 25% or whatever of ability and the stigma for 5 years of being in the school that you have to go to when you aren't good enough for the Grammar) but also want to avoid the non-selective option in Grammar areas, and have a great Comp option if they aren't capable of passing or don't pass the Grammar test.
In most places it's just not possible to have the positives of the Grmmar system without the negatives too.....and even if it's possible for an individual family, it's not possible for the whole area. Sometimes canny families live near the border - they can get into a Grammar if they pass because ey live in the County, but if they fail they can get into the Comps in a nearby COunty on distance criteria, avoiding the secondary moderns. However, those secondary moderns do still exist (whatever people want to call them) and although some are good, lots are not good....but if you have Grammars, some children (the majority) will have to go to them.
The OP is looking for the best option and also to hedge her bets and avoid the worst options. If you like, it's a bit like the 'prisoners dilemma' where in the end, rationale people faced with uncertainty, go for the second best, but guaranteed option - that's a good comprehensive school.
There are lots of good comprehensives who do well for their able students, but also do well for the more middling or less able students too. People forget to look at the data for the ability level of their children and instead just look at overall school performance and decide schools aren't good enough. Yes, there are lots of not so good Comprehensives too and like the Sec Mods, lots of kids have to go to them, which isn't fair or right.
OP understandably wants a good school for her children. Kent has got lots of good schools, but there are also downsides of the Grammar system in terms of the process to get in and the less good education that lots (not all) children get in the non-selective schools which don't have the full ability range - they simply are not Comprehensives. In my view, unless you live there already, unless you are happy to pay as an alternative for school and unless you are prepared to accept a secondary modern education, Kent isn't the place to go to live, because there are loads of other places with good schools where the downsides of Kent can be avoided.