Although I wouldn't trade either child for anything on earth, there is one area that I envy the moms of only children, namely they don't have to walk the tightrope of fairness that moms of more than one do.
Soon after my second child was born, I learned that all children are born with a set of internal scales which allows them to weigh, at any time, the treatment siblings are receiving in comparison to themselves.
This can extend to such things as the age they could date, the size of their rooms, the ages certain possessions were acquired (bikes, laptops) and even who got their favorite meals fixed more often for dinner. Such favoritism (as they see it) is not tempered in anyway by expense or other considerations. If one kid's favorite dinner is homemade french fries while the other's is steak guess who gets their favorite dinner more often? If one plays football while the other runs track, who gets more money spent on gear? Personally as a child I resented sharing a bedroom with 2 sisters while my brother had his own, just because he was the only boy.
The instinct is especially pronounced in eldest children, since they have been there all along and think that the younger sibs always get it better than they did. To some extent they are right, the younger sibs do get it better. There are several reasons for this:
Prosperity. Unless one is caught in the economic downtrend, or have other family issues going on, parents are generally making more money by the time younger sibs come along. This is especially true when there is a wide age range between kids...My youngest sister is 9 years my junior, and my mom worked full time through her childhood. By the time she started high school the rest of us were long since out of the house. Naturally there were more resources available.
Relaxation. Face it, we are more uptight with that first kid. We guard them from everything that can possibly go wrong in the world. We escort them to the school door, interview friends' parents before we let them go to parties, buy everything brand new because we are afraid there have been recalls. We would wrap them in cotton wool till they turned 21 if we could. Firstborns are without a doubt guinea pigs for parents still trying to figure out what works. When we get to number 2 (or more) we relax a bit, in part because nothing too catastrophic happened to number 1.
Holding On. This is especially true of youngest children. The first born we want to see grow up and mature. Often we need them to do so quicker, to help with the younger children. With the last child we tend to hold on longer, keep them a child longer, because they are our last chance to enjoy being a parent. The irony is that since they are exposed to their older siblings and their friends, they tend to want to be treated as older sooner.
Within our own family I hear about fairness issues all the time. The eldest got his first laptop at 14 and helped to pay for it, so his younger sister should have to do the same. The younger child complains her room is too small compared to her brother's. (I tend to not have much sympathy on this one as she at least has her own room. ) We make the younger one go to bed sooner than we did the older one at the same age (the older one never seemed to need as much sleep.) And I wont even begin to get into the endless debate of devotion of parental time and resources between his baseball and her ballet.
The late great Erma Bombeck wrote 2 columns that addressed far better than I the parental battle to be fair. (Both can be read at this link Erma Bombeck on the Favorite child) In one she tells each child the different reasons why they are her favorite. In the other she explains that the favorite child is always the one that needs us most just then.
Of course it doesn't matter to the kids. It never will unless they too someday have more than one child and find out that we always do love each of them the best .