I don't consider this normal behavior, in the video was a bit more tame than usual but other times she will get really snappy and will bark for no reason. She wasn't barking at the item in the hand of the boy. This happens a lot recently, when either child closes the gate, they are near the gate, they go upstairs and sometime just when they are sat in the same room as the dog. Today she did it to one of the boys friends she's never even met before and started barking at him.
When this does happen she will bark and bark for a while and if i try to remove her from the room for a timeout, using the long line she will become very aggressive and begin to snarl and once began growling at me.
Apart from this one issue she is a fantastic dog and is really well behaved, but it's gotten to a point when I don't trust her around the children just in case one day she does decide to snap at them.
I did speak to my trainer about it and they said "without seeing it happen, it's hard what advice to give" but she suggested getting the kids doing training and exchanges with her for her toys and high reward treats.
We have been given a number of reasons as to why this might be happeing; Wanting to play with the children, them leaving her area without permission, children coming near her personal area without permission, protecting toys and food and scared of the children.