If all you want is a Black belt for yourself or your child, then go and buy one, get it all nicely embroidered with your name and any rank you like for about 25 quid. But beware, a belt will not save you, it will not protect you, it will not win a competition for you. Only correct instruction, dedicated practice and effective training will enable you to develop the skills you need to do this.
An unearned belt will make you look a fool, will make your instructor look a fool and will make you vulnerable.
The instructor who promotes a student before they are ready, potentially harms that student. An unearned black belt can give a student, children & vulnerable adults in particular, a false sense of security in their own ability. It could put a child in a vulnerable position with bullies who, may try to push them to demonstrate what they can do, or provoke them into a fight. Then when the child clearly can't 'do', at best they lose face, get laughed at, and their self esteem takes a bashing, at worst a serious injury could result.
If a student enters a competition wearing a belt that is too high for their ability, they will be totally outclassed, made to look foolish, or get injured. Their instructor will also look foolish, and people will question his or her integrity and ability.
Better to remain forever as an OK green belt than be laughed at as a joke of a 'Black belt'. Of course it's better not to stop progressing but to persevere. So what if you take twice as long as someone else to get to black belt, or if you never get there at all. The point is to enjoy your training and always try to improve.
As instructors we owe it to ourselves, our fellow instructors, fellow practitioners of our art, but most of all to our students, not to grade them too soon.
Students / parents, is it really just a belt that you want? Think again:
Enrol; in a reputable, certified school, part of an established association, one with external examiners
Focus; on what you are learning and the enjoyment you get from training, not on what colour belt you are wearing
Parents;, do not push your children to grade! If you really must push, push them to perfect techniques and persevere with training