C.M.
My kids eat a very healthy diet too, no processed junk, dyes, etc. When my daughter was having trouble getting enough calories, I made her shakes with coconut milk, avocado, and vanilla. The canned coconut milk is usually higher in calories than the stuff in the refrigerated section. There are several options of coconut milk without additives too. And, if you refrigerate the milk first, it will be a lot thicker. We have also added blueberries to the mix as well, for a different flavor.