It's a hard time, isn't it? Fortunately for us, the admissions procedure has changed this year so that the over-subscribed high school can no longer discriminate against children who sit the 11+ and fail. This makes the whole "shall I enter him/her for the 11+ or not" dilemma a little easier. Previously you would only enter your child if you were 100% certain they would pass (and who can be that certain??).
DS2 is heading for the 11+ in January. He will have a mock 11+ at the end of this month. Based on the result of that, the school will advise whether, in their opinion, he should sit the exam proper. Based on a similar test done at the end of last term, there is no question that he will sit it. He is very bright (although lacking in self-confidence in things academic).
His best friend from primary will be going to the good high school, so, should the sky come tumbling down and DS2 fail the 11+, he won't be too bothered about going to the high school.
If your dd is as bright as you say, I suspect that a poor performance in the maths paper won't scupper her chances of a place at a grammar. AFter all, if it were just maths that counts, there would be no need to do the other papers. The non-verbal reasoning paper is the one which really shows their intelligence and, if she gets through that one, and performs well in the verbal reasoning, the maths will be less important.