ONe thing I've heard is that the cash registers are not able to differentiate between child dp credits and adult dp credits. So the advice is go through the check out solo leaving your kids at the table with the other adult and they can not tell the difference or if you are not able to leave the kids just go through the register twice, once with 2 adult meals and if questioned say the kids are not eating. Then go through again, yes you might look greedy but there your credits to spend lol
People do try this with success but the CMs are supposed to check your KTTW card. Each card has the guest's name on and specified adult or child, but if you look below that it has the breakdown of your party listed as xA xC (so if you are a couple with 2 kids it will say 2A 2C). I have stood behind people at a cash register who ended up paying for an additional adult meal as they had tried to purchase more adult meals than those on the card. Just somethng to keep in mind.
I attempted to change my 9yr old daughters plan to the adult plan - I was basically told its not possible as the Free DDP offer I had is from last year and it has now expired. If she tried to make any changes to the Plan it would charge full price for my daughter. Its a shame as I think she will be bored with the menus they provide for kids now that I have seen all the menus.
You would think they would honour the offer attached to your booking from at the time you booked.

You have been given some good advice re. the buffets. The kids still get the same snack entitlements as adults.
There are ways you can get informed before you go and make the most of it. It's certainly not portion size you are wanting, it's the variety and the kids menus can be very bog standard at most places, but there are some lovely exceptions to the rule.
Magic Kingdom:
Casey's Corner and the Golden Outpost do not have kids menus. Kids credits entitle them to any entree off the adult menu, any dessert and a drink. Main Street Bakery has lovely sandwiches and savoury croissants and the same applies her so keep that in mind when planning your CS meals.
Epcot
Sommerfest, Yorkshire County Fish Shop, Boulangerie Patisserie do not have kids menus so the same as above applies here. Sunshine Seasons is also a good place to stop. Your kids will select off a kids menu, but they have an Asian food station, make your own sub and a chicken drumstick meal, all of which are served for kids on the kids menu.
Downtown Disney
Earl of Sandwich will allow your child to order off the main menu on the DDP. My DD suffered likes the tuna salad and they had no problems serving it to her on a kids credit. We have eaten there many times so it wasn't just a one off.
Wolfgang Puck Express also has lovely kids meals, great pizza and the kids can have any dessert they like so not the grapes/cookie story there.
Animal Kingdom:
Pizzafari serves kids pizzas. I know you can get sick of Pizza but at least it is a break from the usual nuggets etc.
It can also work to order your kids meal at one place and then take it with you to wherever you are ordering yours.
And share/trade. I don't eat dessert every day and certainly not twice a day so I would give DD my dessert or we would share it.