In general, my motto is "Don't look back." Those days are gone, you can't change them or bring them back, and thinking about them often brings sadness. I do have an occasional wander through my memories, of course, but I try not to dwell on the past too much.
When we long for something like simpler Christmases or other childhood memories, I think we're just longing for the experience of being a child. It wasn't the bath cubes, the annuals, the tinsel, the crackers, "the big present" - in my case, always a doll, which was all I ever wanted - it was seeing everything through a child's eyes. However nice you make Christmas as an adult, you can never recapture the excitement, the anticipation and the joy that kids feel at such a time. I think that the same thing applies to all childhood memories. Emotions are so big when you are a child.
I miss that feeling and I miss the adults who were around me and made me feel safe. Mum and dad, my grandmother, the aunt and uncle who spent Christmas with us...Sadly, I've lost them all. But if I could go back to those days, I wouldn't be with my husband (who was a little boy growing up in Africa when I was a little girl growing up in Britain) and our kids and grandkids would not, of course, exist. I wouldn't want to give up my current family to get my old family back, much as I loved them.
If I let myself hanker for anything, it would be the Christmases before one of my daughters emigrated to the USA with her husband and kids. I miss them and Facetime isn't the same, particularly at Christmas. Then I remind myself that we're lucky to have our daughter who lives here, her partner and her three gorgeous girls. (Not to mention her ex-partner who also comes round at Christmas because he has no other family and we can't leave him on his own!} So I try not to think about what used to be, and concentrate on what is happening now.