Xoanon - I think you know kids change as they grow older and their motivation as well
my kids are both doing GCSE now and neither would entertain idea of being paid to sit an exam!
Have your kids stayed exactly the same in the way they react to you and school responsibilities?
Can you with honesty say that both your daughters sat to do their 11+ practice papers with big smile on their faces and asked for more?
You didn't bribe them, but you must have used some kind of persuasion as that work was done on top of they school hours.
So being persuasive is OK but bribing isn't. 10 yo child doesn't know the merits of Grammar school vs. others. He trusts parent for what is doing and why.
You obviously think it's a great idea and if you don't have anything better to do with your cash then sure, why not.
why do you assume bribing is ALWAYS with money?
On another point....
My kids were in a private primary where they had day 8:30-3:30, both parent's working full time so doing anything extra 11+ stuff was on top of looking after family and the usual homework they had been given. yes, they homework was up to one hour a day as included projects etc.
They had exams in primary, tests and did various activities as those were loved by them.
I don't know how many papers my daughter will sit with her 11 GSCE's in the summer, at least eng lang she's got behind her.
TBH not many unis pay much attention to gcse scores and difference between A and A* at gcse should not be of great concern. Maybe some medical schools look at it and perhaps Oxbridge - but those aren't where most of those kids will go. Besides - what counts is not how well they do in gcse's but how school prepares them for studying A-levels and subsequently how they do in 6th form.
I have first hand knowledge from my kids friend from one of our local school which tests kids for 20% places (locals will know which one I have in mind). Classroom control in most gcse top sets was appalling and she was very upset that most of lesson went on keeping 2-3 trouble makers quiet....