Now, when you say the 9 y/o will be in one of the captains chairs...I guess I am confused. If you have captains chairs in the 2nd row and a full bench for the 2nd, that is only 5 seats for 6 kiddos. Is there an additional seat in between the captains chairs? I have worked in older Odyssey's...that's why I am asking!
How much does the 4 and 5 y/o weigh? To be in a backless
booster, they MUST be at least 40lbs and have headrests behind their heads IF the seatback doesn't support their entire torso and head. If you have a child in a position where their head is over the back of the seat and there is no headrest, you should really look into a high back
booster.
As long as you have lap/shoulder belts in each location, it sounds like you have it all worked out.