Lex, this does seem like a difficult decision, i see where your coming from. They seem happy and are doing well where they are, and thats a very important factor. After all a child being happy at school is a top priority as well as obviously doing well academically.
They have been offered a place at KES because they are at the standard that KES requires. Therefore they must be doing well at the school there at.
However i would imagine that they would need to be reassessed at 13 and would have to reapply the same as others at 13+.
It can only benefit you that they were offered a place at 11+ stage, but i should think they would have to still be assessed when the time comes to apply at 13+ to ensure they are the level that KES require.
I am sure you will come to a decision that will suit you and your DC.