Not all minivans are created equal. For example, the seating room and the cargo space in the Sienna are somewhat better than in my Odyssey and also bigger for seating and cargo space than some "full size" SUVs like the Expedition, Durango and Sequoia. Compare the 8-passenger Sienna to those models side-by-side and there really is no comparison for hauling kids.
Models like the Suburban (and equivalent GMC versions) are certainly bigger than the Sienna, though, and some versions can seat 9 passengers. In addition to the fuel economy (16/22 on 2WD model is the best, 15/20 the worst), they also cost significantly more. While stability control is standard, I think side curtain airbags are optional on some of the 8-9 passenger trim levels. If I recall correctly from the auto show, they also have the drawback of having one of the 8 seats in the front row without a shoulder belt and both center seats lack head restraints (maybe this changed for 2007?). The other drawbacks in terms of safety are the limited crash test information (they lack side impact and IIHS results) and presumably inferior handling and braking characteristics. Still very safe vehicles, but they don't quite offer everything the Sienna does in terms of overall safety- not to mention the lack of sliding doors for the kiddos!
Of course, there is the 2008 Yukon Hybrid, with stability control and side curtain airbags standard and a 3rd row seat optional. Not sure if it is a 7 or 8 seater with the third row, though. I believe it is also the shorter wheelbase so it would be more on par in interior space with the Sienna.