Wow, this could be tough.
IT was hurt by the children because they believed they could hurt it. They believed in the silver and the battery acid. It is a creature of belief- it feeds off of the terror of children and the ignorance of adults, while afraid of courage and confidence.
As adults, they did not believe as strongly in their ability to hurt IT, so the battery acid was totally ignored.
How does this work in d&d?
Well, if you tip the players off it is belief- then they could believe wooden spoons will hurt it, and that will be that. PC's are also rarely afraid of monsters- and dive right in.. not quite the thing IT would be fond of.
You might be better off in CoC d20 or Modern d20- and making a rule that children can hurt it, but IT has high DR against adults. That is the most basic route.
Ok, time for my favorite piece of IT trivia. In the book, it seems they kill IT.
However- in the horrible novel the Tommyknockers, which takes place after the good novel IT, one of the people visits Derry- they see IT for a brief moment.
Writer mess up.. or interesting easter egg? Mwu-hahaha.
FD