Many a mom (and dad) has tried in vain to be "fair" -- giving exactly the same things -- time-wise, finance-wise and emotion-wise -- to each child. But being "fair" in that way isn't really fair. It's assigning arbitrary values to unique individuals. Do you want to be treated as just another of the same item in an assembly line? Didn't think so.
Developing individual relationships with each of your children is not favoritism for one child over another. It is the best way to be fair when you think about it. It's giving each child what they need in terms of individual attention and acknowledging unique strengths and personality traits. Yes, one relationship may "cost" more than another, but can individual value to each child and strengthening your individual relationships really have a sticker price?
Call is "special time" or "one-on-one time" or what you will, regularly doing something with each child as an individual is important, as important as spending time as a whole family. In fact, when each child has their one strong relationship with you as a parent, it may make family time better and easier: Less competing over your attention during that family time.
As with so much, communication -- age appropriate, of course -- about what you're doing and why is important. Everyone needs to understand the goals and the benefits. When kids go through an "it's not fair" phase and can understand give and take, you can talk about how you do your best for each of your children, treating them as the unique individuals they are, not as cookie cutters.
In the intensity of parenting, it can be easy to get lost in an arbitrary definition of fair. Stop being fair and start developing strong individual relationships with your children -- and you'll be fairer than you aspired to be.