I don’t like the vitriol reflected from some of these responses.
However, to the OP, I’m sorry you are hurt by this decision but it is one for the B&G to make themselves. There could be a host of reasons why they have chosen no children, and one of these could be financial - whilst the 11 year old GD is the only one on your side, there could be others on the other side not to mention friend’s children.
I do think that you and your DH not attending the wedding is going to run a lot deeper and longer than the lack of invite for the 11yr old. What might you do in say 10 years time if the now 11yr old decides to have a non child wedding? Are you going to be affronted for the future children of the current bride and groom?
Surely, the best answer is that whichever of the parents is less close to the B&G, they stay at home. Maybe take her somewhere special. The other, along with you and your DH, attend the wedding and reception.
Your 11yr old GD gets a special day, the B&G get the day they want/need and there need be NO long term failing out for a knee jerk overreaction B’s DGP non attendance.
But like I said, I don’t think it odd or strange that you feel hurt/disappointed on behalf of the 11 year old, but pleas don’t make it worse.