Will the JR Pass be worthwhile?

I will be travelling with my family of 5, including a 5 year old in December. We will arrive at Narita airport at 6:30pm. I have booked 3 nights near Disneyland, then 3 nights in Tokyo, then 7 nights in Osaka and finally 7 nights in Hakuba. We will be going from Hakuba to Narita airport on 31 December. As we will be there for exactly 21 days, I was thinking of buying the JR Pass. As it is a considerable amount of money, I just wanted to check that it would be worth it. When in Osaka, we will be travelling to Hiroshiima, Universal Studios, Kyoto and Nara.

Any assistance, or advice, is greatly appreciated as this will be a major trip for us. My 17 year old daughter does have a basic understanding of Japanese, as she has learnt it for 6 years at school. I am hoping this will help us get around.

Thank you!