I would certainly not teach a child to play Diplomacy, at least not as it is supposed to be played.
I have taught my children to play when they were rather little (i.e. 8 and 5) but we made a house rule that says that it is prohibited to lie.
We play some other games that include diplomacy, but we always have the house rule that lying is prohibited. For example, we play rather often the game "Twilight Imperium" like that. The prohibition of lying in such games has the advantage to teach children to formulate, and to understand, the exact nature of an agreement.
For, example, if you promise to vacate a SC without specifying where the "vacating" unit would go, you are allowed to move it even further into enemy territory.
I would never want to play "normally" any kind of diplomatic game with my own children : for me, lies, any kind of lies, mustn't be present in our family.