Well, it would have been a pretty good duel between Snape and Voldemort in any case. I don't think it would have been a childish thing to do, for Snape to reveal himself to Voldemort, no. I think that it would have been a far more accurate representation of the Snape we know now, the Snape we took to be evil for the first six books only to come out as a good guy. Snape risked his own skin for Harry, and could have died in a way which symbolised his significance to the whole series, not some limp, helpless person totally at the feet of Voldemort.