Stamping in Boston.
I am not sure whether the following procedure is applicable for the minors :
Everyone has to sign the form I89 and record their index finger print. The form will be available only in the INS office. Then you handover the passports and the forms to the INS officer once your token is called.
My suggestion would be :
1) Call the INS office(not Vermont, but Boston) and check with them or
2) Go to INS when all the members of your family are available.
Stamping can be done in 2-4 hours on the same day, so you don\'t need to worry about it.
BTW, I did the stamping in Boston and it is peaceful. Only thing: Be there exactly at 7:30 to avoid the crowd.