I had this problem in October and it wasn't pleasant for us. We had our 10 yr old son on the adult plan and our 8 year old son on the kid plan. Our DD 4 was also on the kids but she could be happy eating mac and cheese every day, so she wasn't a problem at all.
Our 8 year old was really, really unhappy at the kids choices. He had to watch his brother eat filet, pork, etc while he had the kids choices at TS meals. The appetizers were really a problem for him too. The kids choices almost everywhere was salad with ranch dressing or chicken noodle soup (the SAME chicken noodle soup). Our older son had the adult choices. The desserts were a problem too.
The worst was the CS-two really bad kid choices at every CS place. I actually thought my 8 year old was going to cry at Pecos Bills while he was reading the kids choices-I think it was chilled chicken and sloppy Joe with grapes. His brother told him that he was getting the bacon cheeseburger-nice brother! This was our second CS meal on the plan and I just decided to pay OOP for whatever CS meal he wanted from then on. Same with our DD at Peco Bills-she wanted a burger. I wasn't going to say "no-you'll get chilled chicken and like it!" The CM's at most places let me get adult CS meals with the dining plan for the kids, even though I was willing to pay cash. That was before the seperation of adult/child credits though. I don't know if you can still do that.
The other thing was that the kids meals are really, really small even at TS places. It was enough food for our DD 4, but not an 8 year old boy. Disney has vastly reduced the size of it's appetizers-they really, REALLY are sized for one person now. So, there wasn't much to share there. I don't think we could "share" and have our middle child be happy. Even if we shared desserts, the entrees and apps would still be a problem. I bought him the Kiddy cone at Kona OOP for instance, he was still hungry because his meal a little pita pizza wasn't that great. His dessert was green apples with brown suger which he didn't like and I think the appetizer was a tiny salad covered in shredded carrots. His brother on other hand, got sticky wings and a big steak.
We have decided to age him one year for our next trip-he'll only be 9 and we are making him 10. That way he will be an adult on any
DDP we get. When I booked our May 08 trip I asked the CM if I could do that and she told me that people do it all the time. I know I'll have to pay for an adult ticket for him too, but I don't care. It's totally worth it to us.