Can having an intense interest in a particular subject (ie. Dinosaurs) be a sign of OCD in children?

Maybe, maybe not. Children may have an intense interest in a wide variety of things without meriting any pathological diagnosis. Also this intense interest in dinosaurs would not be enough to diagnose ocd, although it could be related.
Kids behavior. Interest or obsession? Some children can get obsessed with a subject and are very bright about it, know all of its details but cannot get their mind off it, it could be asperger. But a mere interest is not abnormal and is usually a phase.