RGS does not start till Y3.
My DDs went to NPS (right by RGS), the older one till end Y4 and the younger till end Y2 (then to RGS). Lovely school, about 16 in a class, two classes per year. Actually runs up to Y6 although does lose a fair few to RGS at Y3 (but then has intake later on as Gosforth and Northumberland kids are in first/middle/high school tier. Really really liked NPS and would choose again.
Dame Allan's - also lots good about it and know a lot of people who have sent kids there, runs nursery to Sixth Form, junior school mixed and then they split boys and girls Y7 to Y11 then mixed sixth form.
There is no such thing as Central High and hasn't been for years - NHSG is all girls, again have heard good things, runs nursery to Sixth Form, but depends if you want your DDs at an all girls school (I very definitely did not).
Westfield - tiny, don't know how it keeps going but it does, not academic, have a colleague with a child there who moved from state primary where she was being badly bullied and is thriving so they are happy. Too small for my liking when I went round.
You need to go and look round the schools and see which you like. There is a lot of movement between so just because you start at one, you may end up with another.
And RGS is utterly brilliant - it has high academic expectations and vast amounts of extra-curricular activities. Would not be right for every child but is very definitely perfect for my DDs and they are thriving (now in senior school).
SO advice - look round Dame Allan's, NPS, Westfield and NHSG and decide which you like the feel of. Leave RGS to later when you have more of an idea what your DDs are like - then go to the Open Day when DD1 is in Y2. By then you'll have talked to loads of parents and found out what they think of schools because they have older kids. But it's not uncommon to have families with a child in each of RGS/Dame Allan's/NHSG by the time senior school starts as most people pick the school that is right for the individual child.