It's not rude pure and simple. You live in a free society in which store permitting, you may eat whatever your heart's desire. If a strange child becomes difficult because they want one, simply ignore the situation and keep moving on.
Slight hijack apologies in advance. Your question did remind me of a McDonald's ad that ran some years ago. A man stuck in traffic looking bored until the boy in the car ahead of him, start to taunt him with making rude faces ti fill the time in. The man narrows his eyes and then looks down onto the seat next to him at the bag of McDonald's. Next moment he picks up the fries from his combo meal and then starts to eat them, one by one with relish to taunt the boy in the car ahead.
The boy stops smirking and buries his head in his arms in agony, as he can't watch the man eat the fries. The man then is the one to smirk at the end.
So if you didn't garnish the boy's attention and then eat the fries one by one, just to get him upset, then I say again - no you weren't rude.