The concept of "justice"--within the context of relationships--means identifying a person for what he is, using every relevant fact available, and acting accordingly.
This includes a parent identifying certain traits in their children that make them entirely lovable--traits such as the seriousness with which they set upon the task of learning how this world works; their independent, (sometimes brutally) honest evaluations of nearly everything; and their generally benevolent view of this world and the place they (expect to) have in it.
It involves acting according to these identifications (and many more), while identifying that certain facts (such as whether they've eaten their veggies lately) aren't relevant here, and it involves clearly identifying that one's response to these characteristics is love.
"Love," Ayn Rand identified, "is the expression of one's values, the greatest reward you can earn for the moral qualities you have achieved in your character and person, the emotional price paid by one man for the joy he receives from the virtues of another."
Justice also requires acting accordingly, which here means a parent showing their love in simple ways throughout the year and perhaps on special days (like Christmas) giving gifts--as an expression of the love their kids have earned and because, like you said, it's fun to see one's kids happily enjoying something.
In the same way that your love for your kids is a reward, using a standard definition for that term, even though you disagree with it, I also think you show your kids that good behavior is rewarded by others, even though you may teach the opposite.
Again, if the concept of "reward" is used in the sense that Rand used it above, "to reward" means "to recompense" and a "reward" is--as I have defined it recently--"something given or received in response to or recompense for some action."
You may not agree with this definition, but so defined, every "thank you" given your child is spiritual payment for an action taken, as is every smile you give them in response to some achievement of theirs that fills you with happiness, as is your very love for them (let alone the holiday gifts that are but one expression of it).
This is long, so rather than state the material rewards that are equivalent to the spiritual ones noted above, I should say that I think rewarding in this way is an entirely good thing.
Kids need to learn how to evaluate others and how to respond to good and bad actions (or people) in a proper fashion. A child who learns what justice is and how to apply it has a tool for living that'll help him make many good choices in the future, in response to the "rewards" of others. And all this is true whether he wants to live in a bubble or not, whether they do so from a standard he chooses or not, and so on.