I don't know how women with Fibromyalgia function if they have little ones. My FM came to stay after my children were grown and I thank God daily.
May you have blessed days during this holiday season. Take care of you or you won't be able to take care of your little ones. Ask for help even when you think you don't need it. Find a friend or family member who will take your kids periodically so you can rest. An older child can watch the younger one so you can take a nap.
When you cook a meal, double it so you have another meal, that includes salad. If your kids are older, ask them to help with the meal. Making the salad, peeling spuds, and setting the table are a few things they can accomplish. Stop and read to your children, you both get to sit down. While you are sitting, talk about your decorations, Christmas tree, etc. That prolongs your sitting time and you will enjoy what the kids have to say. Older kids can do laundry. The little ones should be taking their own dirty clothes to your designated spot. Heck, you can fold while you are sitting.
If your kids are older, they can wrap the younger ones Christmas gifts and bake the holiday cookies. I know it is something you have always done, but let go of wanting everything to look perfect and LET them! They can all vacum and dust. No it may not be perfect..it's ok. My friend Kristen's son, Alex, loves to vacum. While he is vacuming, she is dusting. He of course being small, wanted to dust too. :D So, Alex dusts the mop boards while she does the furniture.Young children can also make their own beds and put away their clothes. There again, it will not be perfect, but as they grown it does get better. My kids folded the wash clothes and worked up to bath towels. I always redid them..stupid me!! Only redo if they won't fit on the shelf. Here again, they will get better. Depending on their age, little ones can put the silverware away from the dishwasher. My hope is that you can relinquish some of what you do and give them pride in helping.