There are so many variable’s to your question! Many cruise lines only charge half price tips for children. I'd be looking for those!
Are you all going to be in one cabin? I have to think not, because I don't know of any cruise line that has a cabin for 7. If you have two cabins I'd think $50.00 or $60.00 per day per cabin would be okay IF you only ask for "normal" service.
Anyone else have experience with this many children?