I always like seeing the RF walk to and from church on Christmas Day. As Daria has said, Louise first started to attend when she was eight, in 2011, and as James will be eight next year I think it's possible that we will see him next Christmas. As others have pointed out, seven year old boys don't usually stay still for long and church services can be pretty boring for children. However at some churches they have an area where children can sit and occupy themselves with things like colouring in sheets, books and toys whilst their parents are at the service (they have this at my church, and I've been to a few other churches that have this facility as well) so I guess James could've gone this year and stayed in the children's area so as not to disturb the rest of the congregation, but I don't know if they have this at St. Mary Magdalene's.
Regarding the Middleton's attendance, I do think it's a bit odd that they attended but another part of me thinks that it's quite a nice gesture from the RF to invite them to Sandringham. I don't think it's right to completely desert your birth family once you marry into the RF, but I think a better approach would be to do what the Linleys and Chattos do and alternate between years, and if the Cambridges did that, they would be spending an equal amount of time with both families (as I understand it, George has spent more time with the Middletons than the Windsors this Christmas) which is perfectly fair IMO.
I enjoyed listening to HM's Christmas speech this year, though what she covered was pretty much what I expected, however I was very surprised that she didn't mention family events such as Mia Tindall's birth and Catherine's second pregnancy.