First of all... I want to tackle the comments that the RAD mark down people who don't have a balletic body... NOT TRUE!!! The RAD marking system is very fair and does not allow for that. - However that doesn't take away the simple fact that some bodies are better suited to ballet than others.
But back to the question from the OP:
Nowadays, no dance exams are 'difficult' to pass. Yes they are challenging, and only very good children will achieve the highest marks, but all the exam boards are aware that the majority of children are dancing for recreation and are all shapes, sizes and abilities. As long as a child meets the minimum standard for each level, they will pass, and they don't have to be the next Darcey Bussell to achieve that.
The vocational exams (Inter, Adv1, Adv2) are more difficult to pass, but not because they are marked more harshly, but simply because the work itself is much more challenging, both in terms of technique and the artistry expected. That's because these exams are usually aimed at older children who want to take their dancing more seriously.
At the end of the day, you put your trust in the dance teacher to know when the children are ready to take the exam. A good teacher will only put a child in for an exam if they are confident they will pass. If the dance teacher regularly has children failing exams, then I would seriously question how much importance they place on good technical teaching.
As a last note, please remember that good dance teachers care about your child's exam result just as much as you do. We have all been in the situation of standing in front of a class the week before the exam and being convinced that everyone is going to fail. The good news is, when the day comes we're always proved wrong, the kids sail through the exam and everyone is happy... 100% pass rate intact!
(Ps. Just to clarify, I do not believe ANY exam board is harder to pass than others, RAD, ISTD, IDTA etc. Each exam board has their own marking system, so you shouldn't really compare the marks from one to the other.)