Welcome! This forum has been invaluable - so hopefully you will get the help you need.
Agree wholeheartedly with Debims & Watermelon8 that mock tests are a good idea as they will help you know where your DC might need extra practice. Also, it helps them to be in a room with other kids, get used to the timings that are used in the actual test so that they are familiar with the format and that aspect is no longer a mystery on the day when they will have other things to think about!
Vocab is very important, so read lots! There is a good website you can use to practice vocab which you might find useful:
freerice.com
and there is also a maths website where you can practice 10 questions for free each day - after that you would have to pay - so why do more...?

The student can pick the topic they want to practice
https://uk.ixl.com/math/Over the summer hols we did 15-20 mins most days but not at the weekends and not when we were actually away on holiday! Usually in the morning so that firstly, it gets it out of the way, and secondly the actual test is in the morning so you they get used to working on the sorts of questions at the same time of day. The summer hols is the most difficult as they are rightly on holiday, but as the 11+ is so early in the autumn term, it is good to keep them ticking over. Also, nearer the time, we used the 11+ practice books that were timed (CGP also do books that are 10 minute tests and we used to help them get quicker). It wasn't our DCs ability that was our hurdle, but more their timing of answering questions. In practice my lot would take too long over one question and then they didn't answer enough of the paper to pass. They had to learn 'exam technique' - so being confident to skip over a question they struggle with and answer the others - and only going back to the missed out questions if they had time.
I don't mean to sound scary - but it is hard work and commitment from both the parents and the child. When ours had meltdowns (and they often did) we always said that they could stop the 11+ process and go to the (very good) comp we live near. We would be happy with whatever decision they made. In each case they wanted to go to the particular grammar school, and that helped us get through those harder moments. Worth persevering, but it is work and commitment from the whole family.
Good luck!!