Yeah, that’s a valid concern, which we have considered. In short, we’ve decided we aren’t going to get hung up on gifting equal amounts to each of our 3 kids. If the other 2 get bent out of shape over that, that is their prerogative.
More detail, all 3 of our kids are married and employed w/ no debt other than house mortgages and possible car loans. The other 2 kids live in/near Denver. Both of those couples have indicated that they do not intend to have kids, and AFAIK have taken surgical steps.
My kid with the grandkids is easily the least well off (tho solvent) of the 3 families. Both Denver couples make considerably more, live in much bigger/nicer homes; one makes considerably more than my wife and me. We gift each kid a decent (IMO) and equal amount on their birthdays and xmas (tho we also gift the grandkids, so that family gets more.) I’m confident it means A LOT to our local kid in terms of what they are able to afford and do for enjoyment, their house, purchases, travel, kids’ experiences… We often wonder how much the 2 in Denver even notice our gifts in terms of their budget. One of those 2 is even pretty bad at even saying “Thanks for the check.” And to the extent we hear of their spending, they seem to have few limits on their ability to buy/travel pretty much whatever they want.
And we have college funds for the grandkids. I dunno - maybe someone might think we should gift the 2 childless couples similarly. We are not holding anything against them for deciding not to have kids. Their lives, their choices. If they chose to have kids, we would establish college funds for those grandkids as well.
We periodically go to Denver and they come to us, on which occasions we gladly foot the bill for whatever. In the past we have taken them on ski vacations (which their spouses did not enjoy). We are open to any other travel suggestions they might have, but they have not mentioned anything. Given the interests and personalities of our various kids and their spouses, we have been unable to come up with any one group family vacation that all would enjoy. Say renting 4 condos or a huge house near a beach, in the mountains, etc.
And we’ve clearly indicated that if they need a sizable loan/gift to buy a new home, remodel or whatever, we are available. Our wills have all of our property divided equally to the 3 kids.
So, in short, our attitude is that we wish to be generous towards all of our kids. And we’d just as soon give them stuff while we are alive - and at younger stages in their lives where they might need/appreciate it the most, as opposed to leaving it to them when we die and when they don’t need as much $. But we aren’t going to worry overmuch about gifting to them equally.
We have long joked that my wife’s “disability” is that she lacks patience. To which I suggest that a “reasonable accommodation” by the world would be that she should never have to wait in lines! 
It is funny. My wife just came up with the idea of us not going last night as she was reading a bunch of DW books she took out from the library. The more I think of it - with the help of this discussion - the more the idea appeals to me.