What kind of van are you getting?
We have a newer Odyssey which doesn't have the standard captains chairs - but close. Before this we had an '08 8-pass sienna which was similar. Both times I had a seat in the middle of the row and a child could easily get in to the van, scurry across the almost-captain's chair, and get in the middle rf seat.
If the seat was actually on the captain's chair right inside the door, I am not sure how it goes.
If you are getting a an ody or a sienna, new or old, this should work for you, The older ones even if they weren't 8 pass you could put the captains chairs side by side so the child could use the one that didn't have a seat as a means to climb into the one that did. The newer ones most are mostly 8 pass and while you can't put many if any seats in the middle of the new sienna you can use that seat as a step up still. Not sure how it goes with the traditional 2 captains chairs that only have one position and no center ... but kids are crafty, I wouldn't be surprised if they can get in. In my old van I had one behind the passenger seat, one in the middle, and the drivers side open or with a booster... my girls would climb over all the seats to get to the one on the passenger side so we never had to climb right inside the door.