My 11 year old son was the only boy in his school to pass the 11+ and qualify for a grammar school, and there was only one girl who did the same. It is rather difficult to get in, but very worth it to try.
My son was 8 when we moved here, he entered school at Year 4. It's likely that your 8 & 9 year olds will have plenty of time to *prepare* for the 11+, which is in Year 6. Your oldest, however, will have passed the exam time, most likely.
Still, because of the circumstances, you can see if a grammar school will give him some sort of entrance test. My oldest was already grammar/secondary school age and that's what we did. Unfortunately, his Florida education was, um, lacking in certain areas and whilst he is an intelligent child, he just didn't have the educational background.
He went to a *normal* secondary school, got 5 GCSE's and is now studying for his A levels in Sixth Form (including a double in Science!). Hard to believe he's off to university next year! Goes quickly!!
They may look at your kids' records, but I brought my boys' records and no one asked to see them at all.
For your 8 & 9 year olds, you can help increase their chances by just being a supportive, involved parent. We were lucky in that whilst Year 4 was a bit wobbly as his teacher was out a lot of it, the youngest got a star of a teacher in Year 6, when it has REALLY mattered.
One thing I will offer from our experience - my youngest hadn't hit the years in his Florida school where emphasis was placed on handwriting. Here, the school he went to started it almost from the beginning. So he's still about a year behind in penmanship, and it colours his assignments. So maybe take the summer to really work on penmanship with your kids, if the schools haven't already.