Sentence case in a message

Solution 1:

For English grammar, I think it should be: "Wrong answer. Try again."

However, there's possibly a technical argument in graphical user interface design that says that title case with little punctuation is more effective in feedback to users. That discussion would be best for another forum, however. Perhaps in User Experience?

Solution 2:

Although you're teaching word recognition, the whole experience should be useful to the child. So the text certainly should not have an initial capital on every word [is that really "sentence case"?] because that's not how language is used normally. The message is not a newspaper headline.

I'm not sure what age-group your game is aimed at, but I might think twice about the rather bald "Wrong answer" and prefer "That's not right" instead.