C.C.
Hi from Brooklyn!
Oh my - Sugar Factory for 12-year-olds?! No way, no way. At night it's basically a nightclub (unless you mean during the day - lunch could work). The amount of chaperone eyes you will need at night there will be off the charts, especially if you have a big group of girls and some will want to "dress sexy".
Let's go for "fun", not "happening". How about Serendipity III on the Upper East Side for sundaes? Or the Melting Pot, there's one in Hoboken? Or afternoon high tea at The Plaza Hotel?
(And yes - Sugar Factory will be a crazy bill, just way too much all around for 12-year-olds.)
Bottom line - avoid the Meatpacking District at night with a group of 12-year-olds. Use that "rule" as a starting point.