During the 1st weekend of every month, Bank of America is offering free museum admission. Simply present your bank of america credit or debit card at ticketing and get in free to museums across the country. For Phoenix - you can use your B of A card at the Arizona Science Center. Hours are Sat/Sun 10AM-5PM.
More information can be found here.
The free dates for the rest of the year are:
Feb 7,8
mar 7,8
april 4,5

Note that admission is valid only for the cardholder so you will pay for anyone that doesn't have a Bank of America card (including the kiddos).

Enjoy!