Child.elm

module Child exposing (init, update, Message(InterestingMessage))

init = 0

type Message
    = InterestingMessage
    | InternalMessage

update message model = model

main.elm

import Html
import Html.App
import Child exposing (Message(..))

main = Html.App.beginnerProgram
    { update = update
    , model = Child.init
    , view = \model -> Html.text "hello"
    }

type Message
    = MyMessage
    | ChildMessage Child.Message

update message model =
    case message of
        MyMessage -> model
        ChildMessage InterestingMessage -> model
        ChildMessage x -> Child.update x model