Jessica Simpson is finally explaining why her kids were riding economy while she was chilling up front in first class during that awkward flight with ex Nick Lachey … and according to her, blame Grandma!

We caught up with Jessica at the airport and she was cracking up over the whole thing — insisting the seating setup wasn’t her doing because her mom booked the Hawaii trip as a treat for the family.

Clearly, Mama Simpson‘s first-class money only stretched so far … but hey, a trip was still a nice gesture at the end of the day!

TBH, economy may have been the safer option for the kids considering Jessica unexpectedly shared the first class cabin with ex Nick — we even told you, it was so awks that Jess swapped seats to avoid sitting near him.