The dining plan already includes an 18% tip. Certainly you can leave a little more if you'd like. Bring cash unless you order OOP items.
There is not problem with the group splitting up because each KTTW card contains all the credits for that room ressie (unless you specified that you didn't want credits on the kid cards). The only issue might be YOU keeping track of things if you have multiple rooms. Disney will take the applicable credits off of whichever card they are handed...which might not be the card that purchased the credits if you aren't careful.
If you are on multiple reservations, tell the server so they can write up 2 or 3 tickets (as needed) and apply the credits the way you'd like. FOr instance, tell them "the two of us and the children are on one card, my mother is on her own card." Also let them know ahead if you are sharing any TS meals.