Tokyo-Osaka-Fukuoka-Tokyo

We are planning to go to Tokyo for our Dec 2014 holiday and planning to use 14days JR Pass. My concern is that can we use JR pass for our trips? Is it better to buy JR pass or regular tickets? We are a group of 16 adults and 4 kids, can we reserve any sit if we are travelling using JR Pass?
Our schedule are:
Dec 24, 2014 Tokyo to Osaka
Dec 29, 2014 Osaka to Fukuoka
Jan 1, 2015 Fukuoka to Tokyo

Thank you and waiting for your advice.

Unless you're making a lot of side trips it would be cheaper not to use the pass.

Also, by not using the pass you can take the Nozomi non-stop from Hakata to Tokyo.

If you cut your travel down to 7 days, then you could get a 7 day JR pass and that would pay off quite handsomely. Or if the dates are fixed, you could fly from Tokyo to Osaka, like on:
http://www.ana.co.jp/wws/us/e/wws_common/promotion/experience_jp/
or else try a low cost carrier like Jet Star
http://www.jetstar.com/jp/en/home
https://www.starflyer.jp/index.html
http://www.skymark.jp/en/
http://www.flypeach.com/home.aspx
You could then use a train from then on, or fly the whole way if you wish.
I am not sure of your planned itinerary; Kyoto is much more interesting, as is Nara. Perhaps you've seen them already? Osaka is best at night for Dotonbori:
https://www.youtube.com/watch?v=COJ_MeXbeow
as well as the wonderful night views from the Umeda Sky Bldg and new Abeno Harukas:
https://www.youtube.com/watch?v=xb1wuyAGgyQ
https://www.youtube.com/watch?v=-AyUZmGbW8g
Fukuoka also has a number of nice places to visit. One good source with lots of info on places to see is at:
http://www.thejapanfaq.com/fukpic.html
https://www.youtube.com/playlist?list=PL7LcPcmoSg-j4RLa6iC7ayvBAbOn8-0...
And by all means if you can, go visit the Nanzoin temple just 20 minutes from Hakata Stn. It's free and one of the best (yet nearly unknown) places in Japan:
https://www.youtube.com/watch?v=lT4VQFmHFyM
Plus look into a Fukuoka Tourist City Pass:
http://yokanavi.com/access_tourist_eg
and be sure to try the Hakata ramen and mentaiko (spiced salmon roe).

I am not too certain, so i am asking.

You are 9 days in Japan yet are using the 14 Day pass?

When you write Tokyo to Osaka. Do you mean you are staying in Tokyo till the last day when you transfer to Osaka to sleep? Do you mean you are undecided on your travel plan? Do you mean you are slowly traveling between Tokyo to Osaka (e.g. Tokyo, Kamakura, Atami, Nagoya, Kyoto, Osaka?)

If you are only traveling out of Tokyo and not returning to Tokyo by train, the 7 Day pass is NOT worth it.

If you are making side trips the 7 day pass might pay off. 14 day surely no; unless starting from Kagoshima and ending up at Asahikawa.