GCSE results day and therefore A-level sign-up day.
So with that in mind I just thought I'd post the table of conversions from GCSE to A-level results for maths to show that there is a strong correlation. This is because A-level Maths basically starts with a quick review of grade 8/9 GCSE algebra content and then gets more difficult. Students starting with a 7 will have to work very hard to keep up, students with a 6 often flounder.
There are sixth forms and colleges that will accept students with a 6 onto A-level maths, but from many years of experience, this is not a good experience for the student. Many will drop out and switch courses early on (and therefore not appear in the results table I've posted), but some will struggle on for 2 years and then come out with a grade that really doesn't help them with university applications.
If you take A-level maths with a 6, even if you manage to complete the course (and a lot won't), about three quarters will get a D or below, with E the most common grade.
If your child is considering maths with a grade 7 but will be applying for extremely competitive uni courses, consider whether they actually need it as A*s and As from a 7 are not likely. Obviously, anything is possible, check how far they were off an 8, ask their teacher for advice, but please don't assume that an (old) A at GCSE translates to an A at A-level.
This data is from 2019, there's more recent data but not in as clear a format, and the message is basically the same.
If your child is keen to do maths, but gets a 6, please consider Core Maths instead, which is designed to be taken alongside 3 A-levels and provides supportive maths content for A-level sciences and social sciences (e.g. psychology, geography).
If your child is going to do A-level maths (particularly if they are on a 7 but also 8/9), check that they have done summer bridging work so they hit the ground running in September.
If they haven't been set any bridging work - don't assume that's ok! There are some resources here to help https://alevelmathsrevision.com/bridging-the-gap/