<p>Is anybody using JSON.NET with nHibernate?  I notice that I am getting errors when I try to load a class with child collections.</p>

Are you getting a circular dependancy-error? How do you ignore objects from serialization?
Since lazy loading generates a proxy-objects, any attributes your class-members have will be lost. I ran into the same issue with Newtonsoft JSON-serializer, since the proxy-object didn't have the [JsonIgnore] attributes anymore.