I give this website high marks. The idea of trying to get kids to explore and find their passions early is something highly commendable, and frankly more and more necessary in today's world. Unfortunately, thanks to the sheltered existence many children have and the education trend de jour, much of our youth go underexposed to learning experiences as basic as making pancakes or drawing something they imagine. DIY is a great way to introduce kids to things that they CAN do if they want to. The self guided aspect allows them to move themselves forward and gain drive and appreciation for the skills they develop, and as they grow older, ideally they will seek other skills to practice.
Nervousness about the self-guided nature of the website is to be expected, but DIY is quite transparent, and you can keep up with what your children are posting through your own account.
Consumerism is noted because DIY does make and sell badges, which can be very tempting for your maker. On their own they are not particularly expensive, but they can accumulate, and your children might pester you for them. Explain to your children that they can get the badge once they've earned it (either through the site's tracker or your own standard)
All in all, DIY.org is something I wish I had been able to use when I was a more appropriate age for it. The site is geared towards 10 through 12 year olds, but an open-minded teenager who doesn't think it's lame because it isn't edgy will do just fine. Speaking of which, all posts must be cleared by the staff before they are published to the site, so while there is nothing stopping an adult from posing as a child, both DIY and you are able to moderate interactions with others. Essentially, if you keep up with your kid's activities, everything will be fine. It's a fantastic experience for your children, and a great opportunity to do something together.