I couldn't find anything with a lower saddle height than the Rothan (which is 30cm), but that was over 3 years ago now, so others might be available now.
He started on it at 20 months, and he's never been that tall. He moved onto a Cnoc14 at just over 3. That was the lowest saddle height bike I could find at the time too, and he was just big enough. So we got 18 months use out of the Rothan, he could ride the Cnoc within seconds of getting on it, and he still enjoys scooting about on the Rothan occasionally.
So I'd definitely recommend getting a balance bike first – they're the right size and nice and light, and no extras (brakes etc) to confuse. You'll definitely get your money's worth out of it getting it this early.
Our Rothan has a second rider on the way, and I'm trying to work out how low a standover 20in wheeled bike I can find for it's first rider – that's still looking like an Islabike too, but he's not quite there yet (he's 5 now).