Kettler Air Navigator Tricycle
My husband and I just put this trike together and had a similar problem - the front wheel always turned the pedals and we couldn't figure out how to stop this. The manual for the trike is awful and very hard to understand. After spending a LOT of time analyzing the pictures and doing all sorts of weird things to the pedals, I found a posting on a website that said they'd had the same problem, but had removed and re-attached the front wheel and the problem was suddenly fixed. Sounds too good to be true, right? Well, we noticed that in one of the manual's pictures that the air valve for the front tire was facing the other direction - we'd actually put the tire on backwards. I took the tire off, put it back on facing the other way, and suddenly it all works as it's supposed to.
The thing is, you actually don't have to "do" anything to change from free to fixed and keep the pedals stable. If my daughter pushes the pedals around, they move around, and if she keeps her feet steady on them, the wheel continues to go around while the pedals stay put. It truly is automatic, which the diagram in the manual does a terrible job of conveying.
Hope this helps someone. We like this trike, but the directions are beastly!