Kazzy tells you what the perfect snack is.
Parents who give their children the Polio Vaccine are unfairly disadvantaging children whose parents are anti-vaxxers.
It's possible for EVERYONE to have the vaccine. Where we get into "unfair advantage" territory is special, perhaps even unique, schooling/experiences/training etc.
A great example would be Obama (yes, I know he was joking) using NASA to "help" his 8th grade daughter do her math/science homework. More realistically, he had the money/connections/resources to have high level tutors or whatever.
My third daughter is in 8th grade. I help her do math/science multiple days a week. We're currently into functions and balancing chemical reactions. The challenge "balance" problem had 7(?) unknown variables, I found it mildly challenging... but I do "evil" Sudoku for fun. My wife teaches college Physics.
So my kids have high level STEM tutors at their beck and call. If we're going to define access to high level STEM tutors as an "unfair advantage" to the point of banning elite schools for the rich, what do we do about my kids?
And this whole STEM tutor thing is just the tip of the iceberg. There are huge advantages to being married (as opposed to single parenting) in terms of specialization, diversity of skills, more time, money, and so forth. There are other huge advantages to instilling realistic thinking on the subjects of money, planning, education, math, etc. And yes, there are "advantages" gained by reading to your kids at bedtime.
Which means any kid who doesn't have these things is disadvantaged.
In theory, SJWs want a world where someone born into the bottom quintile has a 20% chance of making it into the top quintile. That's a real challenge given different levels of parenting. IMHO it's an impossible challenge short of taking all the children and letting the state raise them.
There are things we could do, and maybe should do, to narrow the gap... but at some point my kids are in competition with others for limited resources.