"For my grandson I imagined a small sailboat perhaps ?
It needs to be simple but robust."
Hmmm! Just a thought ... I get one now and again😁
Might be simple for you to build Rob, but maybe not so simple for the grandson to control?
If a kit, how about something like this?
A good planking exercise for you, and you can leave off some of the more fragile details if you wish. But I suspect that you may find that difficult😉
Cheers, Doug 😎