Amanda, thanks for the great question. It sounds like you were able to snag some of the best reservations for your little princesses! They are sure to be floating on a cloud with all that royal treatment.
Now, in regards to your question, I think either are fantastic choices. I have eaten at Akershus but it has been years. However, many have told me that the food is even better than at Cinderella's Royal Table. Many of the same princesses will be at both.
In regards to Garden Grill, that would definitely make for a fun change. This is one spot that I am really wanting to try, myself. In fact, I am trying to work it into our dining plans for our summer trip. Once again, I have heard nothing but good things about the bountiful and fresh menu here. The atmosphere is a unique on in that the entire dining area rotates (very slowly) and gives you little glimpses of the different scenes from the Living with the Land ride below! I have also been told that the lower level of the restaurant is better if you are able to request that (be advised that all requests are not guaranteed). Many of the vegetables and fruits used in some of the dishes are grown right there in the Land pavilion.
While both Akershus and Garden Grill would make fantastic experiences, I would maybe suggest Garden Grill to do just as you said, "mix things up" a bit!
Have a magical trip!
~Allison L