In my experience kids will watch almost anything as long as it's colorful, it moves and it makes funny sounds. Surely Angry Birds animated shorts are popular because kids recognize the birds, but I'd say that's hardly a basis for a strong, stable and lasting brand. When some other game with cute characters gets very popular and kids lose interest in Angry Birds games they (and by transitivity: their parents) will also lose interest in Angry Birds animated shorts and Angry Birds merchandise (exhibit A: this news article). It's not like <12 year olds are the best target audience for brand loyalty and disposable income.