I would never bring my kids to a 9pm dinner. We're in bed by that time. Add to that the fact that it's pretty much unheard of to be seated on time at CM, figure you won't be out of there until maybe 11pm if you're lucky (assuming you want to see all the characters which I assume you do) and then you still need to get back to your resort before you can begin getting ready to go to bed. Are you staying on the monorail or will you have a car or will you spring for a cab? I can't even imagine getting back to your resort at this hour without one of these options (if staying at a WDW resort you can take a bus from CR to DTD then to your resort but boy will that take a long time and with a very sleepy possibly melting down 4yo I wouldn't suggest it).
Is your 4 year old used to being up late like this? If this is a regular occurance for him then you can consider it but if it's not then you're setting yourselves up for a major meltdown.
My short answer is keep the 11am ADR and drop the 9pm one. You'll be SO happy that you did.