Nope, it is not "OK" for a mother to take her child's food away without asking permission first. This isn't a matter of parental authority - it is general politeness.
Let's say you were eating with a married couple. The wife orders an appetizer, and when it arrives, as she starts to take a mouthful, her husband grabs the plate and asks, "Does anyone want some of my wife's appetizers?" Would you not think, at a minimum, he is being rude to her, in not at least asking, "Hey, can we share those?"
If the family tradition is to share appetizers, it does not spare the mother from the minimal common courtesy to her child to ask, "Hey, can we have some?" Basic politeness requires you to ask such a question, even if the expected answer is "yes, of course".