Too severe? It's a question of etiquette, and even of protocol in the presence of the most important office-holder in the US.
Now, imagine that you were a middle manager in a large corporation and the Chairman of the Board came to visit your group. While the CoB was talking, would YOU whip out a cell phone and call your significant other? Of course not! You wouldn't do it out of simple courtesy (and obviously because you wouldn't want the CoB to be annoyed at you). I would think the same courtesy would be due the President of the US.
Bad manners is bad manners. I have children, too, and I would be horrified if my daughter did what Sasha did. There is a time and a place for phone calls, and that was neither the time nor the place. Mind you, this is not a knock just on Sasha. Other US athletes had their cell phones out and busy too, and they were rude, too—it’s only that Sasha took it one step further.