Christ, Rob.
Overall, I agree with you (Rob) on this one. Give the children the facts, as straight as possible, and let them decide what they should/shouldn't be proud of.
And Iljets, the 'American Empire' as they call it, never really expanded to its full potential, because the Spanish and English were both there on either side of them.
That wikipedia article might be sort of misleading, if they are counting what the British
claimed was theres, as opposed to what they really controlled. Heck, they said that any land off of the Mississippi, or tributaries of the Miss, or anything connected to that, was their land. Well... that wasn't really the case now was it. That was only their statement given their belief that they could claim any lands owned by the 'savage' Native Americans to be their own.
But hell, lets not turn this debate into a discussion of who was a larger empire when and where... thats impossible to measure if you are counting anything important such as global power and influence, and 'Market Share'

Size is relatively unimportant when you consider other factors of what makes up the "Greatest Empire".