I see what the situation is - however, the DIS doesn't advocate violating Disney policy by using child credits to purchase adult meals. I can't see there being a problem with using adult credits to purchase a child meal as Disney's not out any money on that one.
If your cousin's kid is not with his own family how would the kid use the dining plan from his own reservation to purchase a meal? Would he use his own key card? Or are you asking if your husband can pass your cousin's kid off as your 3 year old and you can pass the 3 year old off as the cousin's 12 year old? Or could your husband get the adult and kids meal from your reservation, and your cousin order the adult meals from hers? As long as the proper credits are being used to purchase the proper meals, nobody is going to care who actually eats the meals.