FWIW, I read the invite as saying that afternoon tea is included in the *time frame* of the party.
eg. 'the party will go from 1-4pm, during which time we will have an afternoon tea. Please bring something to share'

It is really badly worded all round, but the rest of the issue, I personally, could shrug off. Kids wouldn't worry about things like this, and they definitely know how to live a more fun and stress-free life LOL