I did a DIY job on my eldest and he got through - he got top marks in his SAT's too. My second child would just look at the NVR questions and say, 'It's B' (and it was!). I'd be frustrated because eldest child would have said, 'ahh yes, rotate once to the left, flip over, turn inside out, add on 7 and divide by 11 and the answer is B.' This was so much more reassuring!
I discovered that my second child has the exact same learning capacity, but a whole different approach - far more laid back and prone to laziness,
so whereas 1st child needed stimulation, 2nd Child needs motivation, but both achieve really good results. Third child is a mixture of the two!
Recently 2nd child (in top group for maths) took a calculator paper and came bottom of the class! Big brother went over the paper, explained stuff - short-cuts, methods etc, and last week the class did another calculator paper and he got 33 out of 35 - 91%; top in the year group... It really wasn't that he couldn't cope with the work, it was that he wasn't motivated!
I'd let your second child go for it, (being keen to take the test is half the battle it seems - I'd rather have a child looking forward to it than one who is dreading it!) but like all kids, they need to understand competition is fierce and not to take it for granted that they will pass.
I kept the scores for my 1st child's practice questions and compared them along the way to my 2nd child's and they are virtually the same, and at times (suprise surprise) my second child even excells in correct answers. Maybe this is because the tutor (aka me!) is getting better!