Welcome Aboard, Brandy!
On a Disney Cruise, you can choose who has a Key to the World (KTTW) card that has charging privileges. Unfortunately, while you can "load" the Disney gift cards onto your shipboard account, you cannot designate how much each person would get to spend, ensuring that they are only spending their own money.
On our recent family cruise on the Magic, my children were thirteen and ten years old. They were told that they could charge one smoothie or special drink per day and they had a souvenir limit. They knew that we could check our stateroom balance each day to see what they had purchased (it is broken down by each person) and that if it was abused, we would remove their charging ability. They both did great, I think it made them more conscientious about what they were purchasing.
Your other option is to NOT give the children charging privileges and they would need to find you in order to buy anything. We did it this way when our girls were younger, and the only things they really wanted to buy were souvenirs. You can pay in most of the onboard
gift shops with a Disney gift card as well.
I hope this helps with your planning and that your cruise is nothing but smooth sailing!
Happy Cruising, Tracy V.