Just took my two kids on Oct. 6. DS4 and DD 11mo. Obviously it didn't matter to DD. I wouldn't do before you have done something more kid oriented. I think that's where we made our biggest mistake. I think your two DDs will love it, especially if they like dressing and frilly "girl" stuff. It's really hard to say about your 8 yr. old. My DS did fine, although we had to really coax him to stay on good behavior the whole time. We bribed him with MK afterwards. You might consider letting your DW and DDs go the tea while you and DS do something else. But if he's good and sitting and behaving quietly for an hour or so, he'll probably do fine. Hope this helps.
BTW, if you do the full course tea, be prepared for the fact that it's a LOT of food, don't think of it just as a light snack or something. By the time you have the sandwiches, scone, and pastries you are absolutely stuffed. Next time we will just do tea and scones a la carte.
For the kids they don't get the scones with the Mrs. Potts tea, which IMHO is the best part of tea. Mrs. Potts comes w/ choc. milk, 3 half sandwiches on white bread, (PB&J, ham & cheese, and tuna salad) our server they would sub if the DS didn't like any of these. Also comes with two pastries.
I hope this answers your question, but if you have any more specific question about menu, etc. LMK and I'll be glad to try to help. Tea is something my Mom and I never miss when we go, but I don't think we'll take the kids when we go in Jan. DH will stay at hotel with them.