If the gifting is really so significantly lopsided that the children can't help but notice, then I'd avoid shared opening of most gifts; do ours at home and take my children over later to connect with extended family.
Kids will often take their emotional cues from watching adults. If you are genuinely happy for what your children DO receive, they probably will be, too. They may not have any social expectations about deserving equal gifts with any other particular family members until they hear a parent fussing about it.
After all, parents in some families might give their kids much more expensive gifts than they gift each other, and the kids wouldn't question that – it's just the way it's done. Likewise, in some families, a husband and wife will give each other a much more lavish gift than the kids' presents, and if that's traditional, the kids won't question that, either.
And even on the same generational plane, grandkids, for instance, some will simply have much greater contact with a particular set of grandparents. More distant family, not as well known, might seem more like friendly strangers. Fortunately for my hypothetical grandchildren, since I have only one, I'll never have to find out how hard it might be to treat grandkids equally. I hope I would show fairness and respect similarly for each child, but I suspect it would be really hard to relate to each child in a way that would seem fair to each other, while still honoring the depth of any unique relationship.
Hmm. Interesting problem. now that I really think about it.