Or it could be because of the church. You really won't know till you talk to her about it.
My son is 12 and doesn't like large busy rooms, buildings or wide open spaces. He gets all out of sorts and cant sit still, wont eat or drink and feels overwhelmed, and stressed. My son is ADHD or maybe he just suffers from anxiety - I suppose in busy open area's I try my best to just clutch him and keep him close to me when he feels "weird" about such environments. I don't forces him to accept anything he doesn't want to accept.
As for your daughter, just talk to her, and listen to her when she speaks. Most kids don't talk in code, if she doesn't like it because it was too big, then understand the church was too big. Try a smaller church maybe she might take to it or maybe she might just be going threw puberty and is rejecting everything she is accustom to and just doesn't want to go to church for a bit. Listen to her, she will be honest with you if you are open to what she has to say.
Reading your post - your son has a lot of the symptoms of Sensory Integration disorder. SIDS looks a lot like ADHD, but is handled in a very different way - and without meds. Check out the bottom post in this link for a feeling what it looks like in kids - and if it looks possible, google it for more info. http://www.rxlist.com/attention_deficit_hyperactivity_disorder_adhd/article.htm#tocb
Is this for the daughter or the son in this post?
She may be avoiding church altogether and is inventing objections. Not everyone is religious and it is during puberty that one begins to make adult decisions. I don't see that it has anything to do with ADD.
Its too the son. I don't know enough about your daughter to have made that suggestion. But I will say that church was always a quiet place to me which I enjoyed. So I did wonder if maybe the style of worship made a difference? And I agree with all mymarbles, I don't think ADD has that much to do with it. For a child with ADHD, yes - it could be very uncomfortable having to sit that long.