I'm going through something similar with my 5 yr old so i TOTALLY understand your concern. Mine doesn't drink juice, etc. He had his check up last week and the dentist said he need 8 cavities. The more I thought about it, the more concerned I got. I ended up getting a second opinion from my dentist. He took x-rays and went through them with me in pretty good detail. He seconded the first Dr's diagnosis in the end. When the teeth are that decayed, there's a risk of an absessed tooth, infection and possibly spreading the decay to the adult teeth.
It just made me feel better talking to my own dentist. The problem with our pediatric dentist is that I can't go back with him so I have no clue what's going on. I see the Dr for maybe 3 minutes and he didn't go through the x-rays like my dentist did. Moving forward, I'm going to request a more thorough discussion with the dentist and the hygenist as well. Five yr olds (and 4 yr olds) don't always do the best job of telling me what happened. I think if the first dentist had spent more time talking to me, I would have felt better about his diagnosis.
You might want to try somethign similar if you like your dentist. The end diagnosis might be the same, but at least you'll feel more sure about the procedure.