We invited some kids and not others, and some people were upset by it, but we did the best we could to be both reasonable and fair - those two things do not always coincide, you know! Nieces and nephews were invited, but that's where we cut off. As a result, most of the kids were from my side, with only one or two from DH's. The thing was, though, if we invited kids on DH's side we would have had to invite cousins' children. On his side that's a couple of kids we didn't know well. On my side - I have 31 cousins. Yes, that's first cousins. Representing a range of something like 60 to 11, not including spouses. Or kids. Or, in some cases, those kids' spouses. Ya gotta cut somewhere.
I think weddings in general would be a lot easier if everyone remembered that this is a complicated party merging two entirely separate groups with their own expectations, structures, and demands in a relatively high-cost, high-profile, high-stress manner, all of which is being run by amateurs. Most of the time, the HC is doing their best and a lack of an invite to whatever (ceremony, reception, rehearsal dinner, etc) is not a commentary on individual value so much as a reflection of the varying influences that guide the guest list.