The Sienna will let you do many 3-across configurations in the second row with the 8-passenger bench. You may not need new car seats. Depending on your seats, you may need a single narrow model like a Graco True3FIt/SlimFit3LX (not the original SlimFit) if you put a car seat in the narrower middle seating position. That would let you put the oldest in the third row, since they need the least help buckling. The 2nd row middle seat is also stowable to allow easier access to the 3rd row if you aren't doing 3-across. Generally, 3-across is a lot harder in the 3rd row because it is narrower. It's more likely when the kids are older in boosters or seatbelts.
The captain's chairs 7-pass model will work too. Most likely, you have the youngest on either side in the second row and the two oldest on either side of the third row. 2021 and newer Siennas have top tether anchors for all spots. Here's my preview that may help-
The All-New Sienna Minivan Has More Standard Safety & More LATCH We know most minivan shoppers will be interested in the hybrid powertrain now standard on the 2021 Toyota Sienna. It delivers up to 36 mpg and offers an AWD version that achieves an impressive 35 mpg as well. In terms of safety...
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