We did this on our trip last week. I bought one adult meal for my 7 and 10 year old's to split every time we ate in the parks.
The kid's meals are SO tiny (like, 4 popcorn-chicken-sized pieces as their "chicken nuggets", and a bag of 8 apple slices - enough food for a toddler/preschooler, maybe), but an adult meal would be too big for one of my youngest 2 kids to eat alone.
It worked out fine...with an adult chicken tenders "meal", they'd end up with 4 decent sized tenders each (about the size of chicken nuggets from Wendy's - some pieces maybe a little larger), and definitely enough fries to share between them. We ordered a cheeseburger for one of the meals and I simply cut it and gave each child half). I brought in applesauce cups and they had those with their meals, as well. We just drank water that we brought in (I don't let my kids have soda very often, so this wasn't an issue for them).
IMHO, the "kids meals" are kind of a joke for any kid over the age of about 4, even though they are marketed to kids 3-9... (so I'd really hate to see the scant offerings that must be included in those "toddler" meals that are marketed for kids 3 and under).