Your kids would be going into P7 and P5 respectively - but would go from being one of the oldest in their year to being technically in the middle of the year. The cut off is 1 March - but because January and February birthdays can defer (as can, but less often, December, November and in theory September and October birthdays), so in practice, September birthdays tend to be at the younger end of the year (at least, that's been my ds' experience, whose birthday is mid September)
One more year of primary for your eldest and then after P7, he'd go to secondary.
6 years at secondary, with National 5s (the equivalent of GCSEs) sat in S4 (number varies according to where you are : ds' school does 8, some areas only do 6 - but have done other subjects for longer so they can be picked up again in S5 or S6, sometimes as crash Highers), Highers sat in S5 (usually 5 for academically able children). After that, technically they can apply to Scottish Unis (I went from S5) but more often nowadays, they stay on for S6 and either do more Highers, some Advanced Highers (equivalent of A Levels - actually worth more grade for grade than A levels
) or do/re-sit Nat 5s or Highers.
Given your eldest's age, they'd probably be best staying on for S6. If you felt socially or academically, they'd be better going into P6, you might be able to argue the case - but I don't know how easy it is once they're into the school system.
Come and join us in the Scotsnet topic - lots of good advice there! 