My daughters Headteacher said when we approached her that in her opinion children should only sit 11plus if they are a high 4 to 5 depending on age at Y5 Sats. My daughter got all 5's. It seems many other parents did not agree with her advice as many had private lessons for their children even though they fell short on their Sats.
In the Heads opinion these children would struggle with the work and she thought it was unfair of the parents to push them into this situation. Some did pass others failed even though they had lessons.
I would go speak to your childs teacher as they will know your childs strengths and whether they would thrive or sink at Grammar.
Some teachers don't know much about the 11+.
My son's primary school in Bucks was adamant that only the extremely bright are grammar school material, whereas Bucks grammars receive about 30% of the county's students. This is because traditionally the school has a very low pass rate, and this probably caused by their refusal to even discuss the 11+ except for the information evening.