DS's doctor told us to pick out a high calorie item that he can have every day -- just get into the habit of feeding him a high calorie snack or drink a couple times a day. If you try getting your child to snack on high calorie stuff and use cream later on in life, they might not like the taste. DH can't stand anything but skim milk 'cuz he says the other stuff is too rich. Growing up that's all they had in his house, skim milk.
Doc also suggested using regular carnation instant breakfast instead of sports shake, pediasure, scandiashake 'cuz it's less expensive and he feels it provides the same benefits. We mix up a pitcher of CIB with whole milk and some cream and keep it in the fridge for meals. When we travel we buy the cans of premade or sports shake, but they seem thicker and have more of a vitamin taste. We get a 22 pack of carnation instant breakfast at the wholesale club for about $6.25 -- at the grocery store it's almost $4 for a 10 pack.