I can't believe the bigotry of some on here. I'm 73 and in my teens I learnt by accident that at the 11-plus I got the highest mark in the whole of my county. I went to an elite secondary school (my parents had never heard of it!) and got a Grade 1 in Maths at age 14. I went on to become an Accountant, and after raising a family I found I was good at teaching so became a Uni lecturer in Accounting subjects.
In my work I have met every possible combination of GCSE levels. A high grade in Maths and English does NOT signify intelligence or even ability. Nor does a low grade indicate that someone is thick. Plenty of intelligent people struggle with certain subjects and Maths is definitely one. My late dad hardly went to school at all but was highly numerate - he learnt through trial and error and common sense. My late mum was highly literate. Dad could barely read, mum was hopeless with numbers. Both held down good jobs, and were superb parents to me.
My best friend when I had kids was a lady who failed Maths O-level. Her 15-year old son also struggled so the pair of them went to evening classes - and both got good passes. Different setting/teacher/level of pressure and in my friend's case a lot of life experience.
I think it's barmy to insist on a particular grade at age 16 in order to progress further. Yes, encourage another try (with appropriate support) but in life there are other ways of achieving what is needed to get along just fine.
A little story. A woman I worked with had superb results from her private school and became an Accountant. Her sister got v good results from her state Grammar School and became a teacher. Their little brother went to a Secondary Modern school and left with few qualifications. In the early 1980s he spotted that ordinary people were turning to drinking wine more frequently than in earlier years. He became a wine importer and a millionaire to boot.
NEVER let your child (or yourselves) consider yourselves thick because of some inability to perform at a certain level set by others. There's more to a person that their Maths grade. As said, I was a wizard at Maths but have barely used 90% of what I learnt then and don't even remember most of it.
I am biased of course but my DGD is SEN (her brother not so) but heck she is bright as a button when it comes to soaking up information like a sponge. Her vocabulary is phenomenal (but she can't spell half the words) and she has an insatiable desire for knowledge. Her school essays are full of imagination and factual stuff - just badly spelt and punctuated. She'll go far in life - as I hope many others will. Confidence is a big help, whatever grades you do or don't achieve.