Hi, Jennifer!
Well, you've chosen two really wonderful experiences for 4-year-old princesses; well done!
Since you can't go wrong with either magical makeovers at
Bibbidi Bobbidi Boutique or dinner at
Cinderella's Royal Table, the key here is to determine if this special treat will be something that you'd like the entire family to experience (dinner) or if this is all about the princesses (makeovers.) In addition, if you're planning on traveling to Walt Disney World when it's exceptionally hot, your little princesses may be just too warm in a ball gown to wear it for the entire day (many princesses choose change into their own gowns at BBB.)
Keep in mind that there are many photo opportunities when dining at CRT, all of which can include your entire party as well. We love to use the professional Imaging Package as a chance to record who was together on that special day, as well as the ages of the kids, too; they grow up so quickly! In short, while we loved the experience at BBB, it's dining at CRT that keeps us returning.
Regarding a party of 8 and your seating arrangements, Guests are often told that their party may be split; however, if we all arrive at the same time, we usually are seated at two tables which are right next to each other. (This is very practical in that if two tables for four become available right next to each other, you'll be seated quickly.) I wouldn't worry about being separated; the
only time we were separated was when another family arrived extremely late for our reservation (we were lucky that we could be seated at all, let alone near each other!)
Send more questions as you think of them, Jennifer; we love to help!