Child seats :
Unless Toyota made a running change, the 7-seat Sienna has 4 tether anchors (both on the 60% part of the 3rd row), and the 8-passenger version has 5 (3 in the 3rd row).
The current Odyssey has 3 std, but 2 others are optional, for a total of 5 (the "plus seat" doesn't offer one). When the current Odyssey came out in 2005, there was no provision for the 4th and 5th anchors, and a running change was made for 2006.
My 2nd-gen Odyssey has the 2 extra anchors added (for more flexibility on road trips), for a total of 5.
The print version of the sidebar I wrote for Car and Driver's 2004 minivan comparo had a chart for child seat compatibility, although the one that was put to print had a typo. Still, you can read it up for some "parenting" features.