A well engineered 16" wheeled bike can be as tough and durable as any other. You'd have to think carefully about going fast off road, but you would on a 700c racing bike too. I think we should be careful about making wrong assumptions about children's toys and small wheeled bikes. Maybe they don't appeal to everyone's idea of what a bike should be. That's fine, but it doesn't mean they aren't as tough as old boots. I met an old guy with an eight year old Brompton the other day out in the middle of nowhere. He'd got on a bus with it in the morning, travelled twenty five miles, and was riding it home. He said he did this about three times a week and that he had put over 15000 miles on the bike. He was at least 65 years old. I think the bike will outlast him easily.