The meaning of the word "not fair" (unfair)

Joel and Jeremy is the admirer of Ryan. To them, Ryan is the their hero, big brother. Ryan has got this habit of saying "not fair" whenever he got an negative answer from the adults. Example: No, you can't go to the park because it's going to rain". Ryan will say "not fair". Subsequently, whenever I said no to Joel, he will said the same thing "not fair". So one day in the car when he did that again, I asked him does he knows the meaning of "not fair", he kept quiet. His Daddy then give an example "not fair means, when I have sweets, I give more sweets to your mummy and less to you". The next day or so, on the way back from the school, Jeremy said "not fair" to me about something which I cannot remember. So I asked Jeremy whether he knows the meaning of "not fair". He also kept quiet. Then, Joel very proudly declared to Jeremy: " I know the meaning of "not fair" already! It means when mummy has more sweets than me!"

I nearly burst out laughing but then how can I explain the word "not fair"for them to understand? Sigh!