Instant pudding, all flavors, made with extra milk and whipped thin before serving. (I'm seeing Heather in the kitchen, with a name change-- BlackBoots instead of Duckboots, lots of shiny chains and studs).

Baby formula is pretty high in fat I think-- maybe add some of the powdered form to other items.... There are also fine-milled instant baby cereals that can be thinned to drinkability with milk or cream, and maple syrup.

Syrup..... chocolate syrup by spoonfuls? Butterscotch, caramel sauce?

Egg custard with a high ratio of egg to milk.... Egg Beaters mixed with cream and rum....

Frozen fruit juices made with 7-Up...

~Susan