It is my DD's first trip, too, and she is also 7. I've only made 1 character ADR and was hesitant to do that. I know you all think I'm cooky...You just never know how they are going to react around the characters. She's watched the video, I've shown her the pix and asked her if it was something she wanted to do and she was ambivalent at best. Every child is different and only you know your twins. You just don't know how they will react to all that is WDW once they are there.
My DD has all the princess stuff...princess dresses, dolls, mini figurine sets, comforter set, the list goes on...

HOWEVER, you just never know how they will react. There seems to be so much at Disney, that I didn't want the meals to be more "overload," if you will. Besides, character meals seem to be all buffets - translation:
a boat load of food. I can't eat like that for 6 days in a row!

So, you obviously have that expense. I got great advice from friends who visit about 2x/year with similar age daughters and after about the 3rd day - eating becomes a chore if you overdo the buffets. There's many opportunities for character interactions that I just chose, for us, not to inundate us at meal times, as well. Just another take on it, FWIW.
Have fun!!!! That's the most important thing. Be flexible and willing to cancel if they seem to enjoy other things and don't want to take the time for a sit down. That's my plan anyway.