The seat for my son is quite big:http://www.kieskeurig.nl/xml/layerPopUp_iframe.xml?url=/product/image/E22A1F3210770EECC125712600518F00.html
And the seat itself fits, but he can't keep his feet in front of him, he has to put one feet on either side of the drivers seat.
My daughter is almost four. She does fit in het seat:http://www.kieskeurig.nl/xml/layerPopUp_iframe.xml?url=/product/image/3D19D1D97F20E19FC12574D300285CA8.html
The idea of rubber mats or boxes is a good one! I use velcro-holders. I also sewed a blanket onto the front end of the cover to prevent anyone form seeing what's insde.
I'm a big Smart fan. To bad my two kids don't fit (at the same time). And I still want a small car. The basic Smart is on sale here for € 9.000,- about the same as the basic Aygo. A basic iQ sels for € 12.000,-. The deluxe iQ (with all the options) will cost € 17.500,- About the same as the top top end of Smart (a Brabus Coupe). I think the Smart will apeal to different people than the iQ. If the sales wil be affected, I don't know and I don't know if anyone can tell just yet. The first genertation Smart was very popular as a marketing-car: fully printed. That was a hype. And the first generation suffered form bad gossip about the engine (but yes, there have been better engines). So the sales dropped from the first big boom, but I think they're quite steady now. I'll ask around on the Dutch Smart forum, maybe someone there nows more.