So, in other words, you're actually looking for an 8 passenger van? I believe that leaves you with the Honda Oddysey and maybe the Toyota Sienna.
The 8 passenger Oddy is really nice, but the middle seat in the second row is very, very tiny (narrow). It doesn't give you many seat options. It's best used w/a rear-facing seat or a booster seat, b/c it doesn't have a top tether anchor and can't be retrofitted for one. It's possible the latest models have fixed that. It also means trying to puzzle two seats together very carefully.
OR a regular 7 passenger van w/captains chairs in the back in which you can move the passenger side captains chair to the center, creating an isle on the passenger side to the back. I think most of Honda Oddys allow that (and have for a very long time) and the Sienna's. As for other vans, I'm not sure. The Chrysler Town and Country may allow that. The Ford Windstars allow you to shift their second row bench seat to one side or the other, making an aisle on the side of your choice.
As for your current van, it's all about puzzling in the third row. That's a tight third row. If you have a shoulder belt in the center of the third row, which I doubt since you said "older van", you could try the 2 three year olds on either side of one kid in a booster. The Radian probably would make things much easier. It's often very tough to get a booster seat buckled next to other seats. Different booster seats will puzzle with different harnessed seats. Asking about different combos here, esp in your particular van, can help a lot, b/c someone here has probably found a successful combination!
And fwiw, if you don't have them already or don't know, Chrysler/Dodge retrofits top tether anchors for free. It should be possible to put 5 top tether anchors in your Caravan.
Does that help a bit?