Equal treatment at different ages.

There should be no question - the 10 yr old WON her prize for the extra, winning effort.

The 6 yr old should never expect free money for something he did not participate in. He will have his chance to be a champion in due time, and no one will be able to take that away from him. He has to learn this early. DIL is wrong to have them share equal praise if only one accomplished something.

I agree don’t leave one out I used to treat my children for an achievement and do the same with my grandchildren.

Being treated equally doesn't mean being treated the same. If you gave 4 people a packet of nuts each in the interest of equality and treating everyone the same but one person was allergic it would not be appropriate. So you do need to make adjustments. In your case I would just send congratulations as an acknowledgement of the success to the 10 year old.

Please come back Valeriekent and explain what your son and daughter-in-law have actually said about this.
Thank you.

When I was about 10 and my sister was 12, we both entered a competition run by Cadbury’s. We both completed the comp successfully enough to get some chocolate, but the top prizewinners’ names were then drawn out of a hat. She won. She was not better than me, just luckier.
The prize was a tennis racket, press, cover, balls. I wasn’t bothered about them, she was delighted. The prizes were presented in Birmingham, in a posh department store, including a four course lunch. Free first class rail travel for my sister and my mum. My Dad was away working, so my mum bought an extra second class train ticket and my sister and I travelled together in first class just behind the driver. My mum bought her own meal in the same restaurant and I sat with my sister for the posh meal.
So in a way my mum found a way to treat us both fairly, but not equally, as she did not buy me a tennis racket. It was a lovely day, and I applaud her thoughtfulness and unselfishness.
What a lovely memory from a very long time ago.

What a wise woman your mother was, Cabbie, and what a lovely memory.