As a very experienced secondary teacher, I beg you, please don't put pressure on your child to get all 8s and 9s and start hiring tutors for them as they go into Year 11. It's not fair, it's not necessary, and it puts far too much pressure on them to the point where they risk burning out and struggling with their mental health. They don't need all top grades to be successful in life, and they have so much to juggle in Year 11 that putting this pressure on them is actually counter-intuitive. I've dealt with so many mental breakdowns of Year 11s over my career, usually caused by over ambitious parents putting unbearable pressure on their children, and it makes me so furious. They are so vulnerable at this age and what they need is lots of encouragement, support and reassurance that no matter what happens, you will love them, be proud of them, and that they will be ok. Please remember that teenagers are not fully mentally developed, they struggle to regulate their emotions and put their experiences into broader contexts. Failing an exam for some is quite literally the end of their world and will cause them to spiral. I can't stress enough to parents how important it is not to send the message that exam results are the be all and end all. You really don't always know what is going on inside your child's head, and you don't know what all of that pressure might lead to them feeling, thinking or doing.
As a guide, it should be a slow and steady build up to the exams in Year 11 - think a marathon, not a sprint. On a week-by-week basis their revision should be consolidating what they've learnt that week, ensuring they fully understand any relevant theoretical underpinning to their new content/knowledge and can apply their new knowledge to enrich and develop what they already know. They should be reading back over classwork and making brief summaries, and highlighting any areas they feel less confident on to revisit and, if necessary, ask their teacher for more support in order to understand.
If they are doing this on a weekly basis, they won't need to be spending hours and hours in the run up to exams revising content they should already know. It'll be just refreshing specific details that can easily be forgotten - for subjects like History and MFL there will be a lot of rote learning they'll need to ensure is fully stuck in their brains - and practising examination techniques.
As I always say to my students - the most important thing is to not waste time revising topics you already know. Lots of kids find it really reassuring and confidence boosting to keep going over subjects they are confident in, churning out practice exam questions and congratulating themselves on how much they know, while avoiding tackling the areas they struggle in or for which they need to memorise a lot of content. They can't allow themselves to indulge in just doing the easy stuff - they have to be strategic and focus on the areas of challenge in order to make the most of their time.
Teachers should be providing revision guides and so on when they get back to school in September, but getting your kids who are going into Year 11 to go over Year 10 stuff as they get to the end of the holidays and make a note of what they feel they need to brush up on or ask for support with, is a good way to get them into the revision mindset as they prepare to go into the final year. I wouldn't stress them out with doing a load of revision over the summer - this is going to be a tough old year and they need to go into Year 11 having had proper rest and relaxation over their summer break.