It makes *some* sense, the highest (easily) climbable point on the Model T is like 3 feet (the back seat rest) whereas the highest climbable point on the Fire Truck is easily above 7 feet and if you have a bunch of kids climbing and rough housing on that thing it's more likely than not someone falls off. That said I said it makes "SOME" sense, for reasons I'm sure are very granular and come from an insurance point of view, but I don't agree with it either.
For example, there's a climbing structure for kids in our local shopping mall that, while enclosed is like 15-20 feet high and any kid can go on it and (presumably) fall to injury or worse and yet it's still there after god-knows how many years. So yeah, I don't agree with the Fire Truck being inaccessible, but I understand it from a presumably very insurance minded and risk-aversion management that Dreamworld is no-doubt focused on after ~the incident~.