It should work, although I have not tried. There is a seat release arm to access the third row, and it tilts the entire seat forward, (bottom of seat and back) so it should work fine with child seat attached. If I recall, one side tilts both side passanger and middle seats at the same time, and the other side just tilts the other side seat. So you could also leave the middle child seat attached.

I would not take my word though, go try it out at the dealer.

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2013 X5 35i M Sport