I tutor maths, and sometimes I spend more time training people to pass exams, rather than how to do maths.
Sadly, the reality of the world is box-ticking, and having to do it they way the examiners want. Sometimes, I've had to train pupils to do it the "prescribed" way. I meet people who do A-level maths who can do mental arithmetic much faster than I can, and do lots of algebra in their heads, but they often do badly in exams, because they don't write things down. These people can be harder to teach than those who struggle with maths.
When I was a pupil at school, I remember being told off for using methods we hadn't been taught yet. For instance, "what is the 8th triangular number?" We were taught to work it out as 1+2+3+4+5+6+7+8. I knew that it can also be worked out at 8 x 9 / 2, but I was told off for doing this.
I think the thing to do is reassure your dc that it's great that they know more advanced methods, and they will be able to use them later: year 7 is early days, and they have to get used to following the instructions, and doing things the way the teacher tells them. The time for taking more initiative will come with GCSE and A-level. In exams, it is vital to follow the instructions. A-level questions often say "By using method xyz, or otherwise..." If it says "or otherwise", they can use any method they see fit.
In most subjects, using initiative and doing their own critical thinking comes in later years, and lots of people find this difficult: up to then, they were spoon-fed which method to use. Your dc will reap the benefits of this later.