Hmmm...."coasters" would equal "mountains" plus CS coaster and IJ. On CS, I'd agree with having them sit together with you behind or in front of them.
BTMRR, you should all three be able to sqeeze in, unless your pooh sized we've done this plenty of times with one average and two small adults.
IJ, you'd all be in one row, so no issues.
Splash everyone gets their own seat.
Matterhorn: You could sit behind them and they could ride together.
SM: like matterhorn/CS
BUT, if you want to ride on those rides where there is side by side seating for two, are the girls old enough to wait at a specific spot? You could all go through the line toegether, then one wait at the exit for you two. My guess is you'll have to que up again, but strategically it could be done fairly easily, especially if you're there multiple days.
Easiest solution: Take a second adult: cousin, friend, aunt, or uncle that enjoys the kids! Then you can take turns.