I am an 11+ tutor.
One thing I say to my parents is that the MUST have 2 whole weeks off over the summer with not a single mention of the 11+. Parents look horrfied and kids cheer!
Why? because they get tired. It has been a long year, first full year in school since they were in year 2. Hot weather, lots of extra work for tutoring, and they are knackered. Motivation is low and they are stale.
They take some time off, and come back fresher and more able to do it. There is also something about how your brain sorts and processes information that means they seem to make a leap forward after a short break.
I also find that on the day they all rise to the occasion and mostly do the upper limit of their best.
the other thing I say to parents is reward effort and time spent, reward them after exam and before results come out. Let them know how proud you are of the extra work and input they have put in. Also whatever your own feelings, make it clear that if they pass there is this good school, and if they don't pass there is this other perfetcly fine school, and aren't we lucky to live in a country with lost of opportunities? And also whether they pass or fail they are still clever, they would not be sitting the exam if they weren't and it is just an exam on one day. Take the pressure off. They are only 10.
Given all that, you are so close to the end that I would go ahead and do the exam. Have a break, really get out and enjpy the summer, come back to it last week of the holidays, with loads of encouragement and have a go. It is good for them to have a go after all the work.
And finally, while I did listen to my kids about which school they went to, they did not choose, I did. Because a 10 year old has no comprehension of how different schools approach exams, or what a difference certain things would make. My ds did not want to go to his grammar school at all. Was quite cross with us about it. I promised him that if he was not happy at the end of year 7 he could swap. He came home on day 1 smiling, and stayed. Made good friends, people that he really got on with and is still friends with now. He told me aged 17 that he was really pleased we had pushed him to do it, and he now understood why.