Definitely break up whenever needed. Make most of the trip about the 2 year old but do not rule out the teenager. Adults are so much more adaptable.
Take cell phones (turn them off when in a queue line or an attraction). Keep the lines of communication well open. From what I can see, you have three distinct parties. Each one should plan or think about what they want to do. If it matches up with someone else, then fine. If not, go out and have fun anyway. After three family trips, everyone now understands that. I still laugh at my mother-in-law last Christmas time. "Can I *please* go with you guys?" Turns out her husband and son (not my husband) were going too slow. She wanted action and said we were younger and wanted to spend the day with us. She ended up with my son, daughter-in-law, and granddaughter while my husband and I had a day to ourselves.
As for what you should do - everything you can. At your own pace with your own group. Meet up when you can - maybe for dinner or lunch or if that's not possible, back at the resort for the evening or for afternoon swimming. There's no way you can all enjoy Disney World by being in a group all the time.