We did a trip of 16 in August, in two different resorts, just like you, four families and two in-laws. Here's my $.02:
Plan early and plan a lot. We did a couple of meals together all 16 of us, plus a couple of meals with just 2 of the families. Our best restaurant experience was the Character Breakfast at Hollywood and Vine MGM. We also got a group picture taken there by one of the roving photographers that are near the entrance. Our worst group dining was the Rainforest Cafe. We were separated by three different tables and it was so noisy no one could talk.
Try and schedule one afternoon or evening just to sit by the pool. The kids will like the down time and so will the parents. Your in-laws will have forgotten how much energy kids can have and won't be able to keep up (and probably won't want to).
Two way radios are GREAT, but you can only use them within one park. You can't talk to someone in Animal Kingdom from Magic Kingdom.
Whatever you do, don't think all of you can do a park together. You will move too slowly to see or do anything at all.
I sent a card to the other families in our group a week before we left. I addressed it to them at their hotel and put the reservation number under their name. Just a note to wish them a fun vacation. Kids love to get mail. Then mail a postcard from Mickey to all the kids just before you leave telling them he's glad they came.
Have fun time!
Susan