You don't say which side of Cannock they will be or whether they are boys or girls or a mixture. I will start with local Independent schools.
Tettenhall College goes from 3 through to 18. Its an independent school and south west of Cannock outside Wolverhampton. All 3 children could go there as its mixed sex.
Abbotsholme is in rural Derbyshire. Its just over the border from Staffordshire in Rocester. It would take around an hour to get there from Cannock.
If you are willing to look in that area then theres Denstone College (age 11+). Its 2 or 3 miles from Abbotsholme but much bigger with far more available to do. Its prep school is in Marchington just outside Uttoxeter but theres a mini bus between the 2 sites so older siblings can get dropped at the younger siblings site.
Chase Grammar School is in Cannock itself. Its fairly small and mainly international students. Its reputation is ok but Ive heard better about other schools.
Stafford Grammar school is west of Cannock. Ages 4 - 18. Has a good reputation locally, my DD regularly plays against them in Hockey matches, generally nice children and high attainment.
Lichfield Cathedral School is in Lichfield but their prep school is on a different site to the main school. However there is a mini bus between the sites. Its a very religious school as its tied to the Cathedral. If you are not religious then I would choose another school.
For state schools the best schools in the area are incredibly difficult to get in to. You literally need to live within less than 1 mile to get a place. However, Avoid Rugeley schools altogether. They will usually have space ( the high school currently has 120 spaces for this year 7 alone and a similar number for every other year group) , but only the Chancel is worth looking at for primary age and that will be oversubscribed.
In Little Haywood and Great Haywood both the primary schools are brilliant. Anson and Colwich primary. They might have space.
In Stafford Berkswich primary school is excellent but you will need to live within 0.5 miles for any chance of a place.
Walton High school in Walton Stafford is the best state high school in the area. You will need to live within 0.2 miles of the school for any chance of getting a place there. In year admissions (the 12 year old) are almost non existent though.
I've never heard anything 'outstanding' about any school in Cannock so I cannot really say.
Your daughter needs to email all the schools she wants to consider and have a look round them.
For me the only independent school, from those Ive mentioned, I would consider for my dc is Tettenhall College with Stafford Grammar as a 2nd choice. They offer the extra curricular activities my dc enjoy and take part in.
However if I had the choice of any in the midlands it would be Repton in Derby or Malvern College in Malvern. I'm guessing they might be too far outside your search area?