I've read only the first 15 or so posts, but agree with most that's been said ... especially Kebo's comments - my thoughts exactly!

While its certainly no laughing matter, one accident is one too many.
I recall a lot of accidents - even recently, with the mandatory safety features modern machines have. For that reason, I would feel obligated to be not a lot more than neighbourly. I would never condone small children on tractors, riding in buckets, on mowers or other machinery, unless they're in a cab with the doors shut. To quote one quite successful businessman I know: "It isn't so much what you say, it's how you say it ... it's all in the delivery". Yes, I'd be somewhat miffed if they were the first words I'd heard from my new neighbour, but to say nothing may also equate to condoning such actions. I'd be suggesting I'm drawing his attention to this because I didn't want to ever hear about a terrible accident to an innocent child when I recognised the danger and said nothing. As suggested my Midnightoyl - it might hit home a lot harder if his wife was also party to the conversation. It may begin a great friendship. It may also avert a horrific accident. You never quite can tell, but he may realise this and appreciate your care and concern. He could also suggest you POQ and DCB. At least bring up the subject gently and diplomatically, then you've done your bit to help, as well as extending the hand of friendship. :thumbsup:
A little of my personal background: Grew up on a dairy farm with two elder brothers, driving tractors sitting in front of one brother from early childhood, and by myself under supervision from 7yrs old. I would plough, slash, pull a trailer full of 24 or 30ft irrigation pipes, spread fertiliser etc from 10-11yrs old. We had only MFs - a 35 and a 135, no tractors in that era had ROPS, but I never had an accident. I look at places nowadays where I used to drive and briefly wonder how, then remember ..... slowly, carefully, gently; and look, think, prepare, execute. My brothers and father must have taught me well.
We have both these tractors here with us, as well as a MF40B loader, all of which my son and both daughters can drive quite capably. They all were well-taught on our ride-on as well - albeit only a 13hp 42" Yardman, and one friend was amazed at how neatly our younger daughter mowed. When I answered his enquiry of her age as 10y/o he declared he "knows 20y/o people who couldn't wield that machine around the way she can ... and that's just the guys!" I laughed, told him to not stand still for too long and still smile about it today. I wouldn't let her drive it by herself until I was sure she was safe - her elder sister would usually sit behind her on the seat as a safeguard.
I feel for every family who have lost a loved one through an accident. Prevention is better than cure. Additionally, we can't rewind the clock.
Work safe, know your capability and stay within your machine's limits. Look out for everyone close, and remember the little ones cannot recognise danger.