It depends on size of indoor. If it's not enough room, then 24-70 (I or II), if you have enough place in there - 70-200 F2.8 (II in case of IS)
I checked with my 24-105 this morning and it seems as though the 24-70 range for me will be my more used indoor simply because of room. I am debating on returning the f4 70-200 for the 2.8 but not really sure the added weight is worth the extra $1k and to only gain a stop still will require fill flash to me. I do not find myself using the 50mm simply because with kids it can be hard sometimes not being able to zoom in or out plus the more light the less my twins are in focus so thats why I am having a hard time finding the best way to approach most shooting environments knowing I cannot get them all. I did great birthday portraits with it at 2.8 but there was plenty of ambient light in the room. Basically what is everyones go to suggestion for christmas morning? Should i get the 600ex and go that route?