Thanks, folks. I appreciate the input.
I live in Guildford and we have a nice purpose-built model boat pond in a nearby park. I've never seen anyone using the pond in my numerous visits. While that can be good, it also seems unfortunate in that there doesn't seem to be a local club. I also haven't found an RC model shop in Guildford or the immediate surrounding area.
At the moment all I'm doing is research (and building a nice workbench in my garage to accommodate our new undertaking as shipwrights). I'm liking the idea of tugs for slower speed and maneuverability ideal for a 5-year old. I want to stay away from anything too "toyish". I'd like to have something that the boy and I can assemble / paint / launch together and he can keep as an adult, much as I kept the model rockets I built with my dad almost 40 years ago. He's young, he'll become impatient and he'll become distracted, so I might want to avoid a full-on scratchbuild unless it's something I can bang out in a few weekends.
Some I'm thinking about:
Graupner Pollux 2 (but seems expensive)
Mowe (as was recommended above)
SHG's "Saucy Sue" (I'm worried that the vac formed hulls might be a bit thin and "cheap")
Eventually, I want to scratchbuild a model of one of the vessels I'm currently involved with. Don't know if I want to make it radio-controlled, however.