There is no charge for tea with Wendy. You do need tickets as space is limited. You get the tickets at Guest Services after boarding. Go as soon as you can, they will run out of tickets. I don't know about the 3 day...never did it.
11 is a bit old, but your kids would probably enjoy doing it once. The 11 year old can always "pretend" that she is there for the younger sister (she doesn't have to admit that she likes it). I'd say it is geared for 5-9 ish year olds...but there is obviously flexibility at both ends of that....
2 boys are chosen to be Wendy's brothers during the production. Afterwards, everyone had a chance for a picture with Wendy, but only those 2 boys were part of the action. Everyone else duplicated the action at their own table. So...yes, your sons might enjoy it....