They brought Michelle's mother to live at the White House in 2009 because their girls were so young and needed someone there since the President and First Lady were embarking on very busy schedules that required them to travel and be away a lot. Having their grandmother there to make sure they got to school every day, etc was a way to ensure the girls kept as normal a life as possible under the circumstances. IMO it was a very good idea to do this, having her there probably provided a peace of mind for Barack and Michelle knowing the girls were always with family. Having her along on these official visits where the girls came along also makes sense, better them to be in their grandmother's company than strangers or Secret Service. Hopefully they did something fun today!