MWM, this isn't an adoption subsidy. One parent is getting social security because their age and because they have a child under 18 they get a check 'for' that child also. This is NOT a payment for the child to spend as she likes, but rather it is money to help pay for the child's upkeep so that the child can stay in school instead of having to find full time employment. It is NOT all for fun spending or the child's choice. It is designed to help the child be ABLE to stay in school by helping pay household bills (the child's share is how it is seen, meaning divide hte rent, utilities, etc... by the number of people in the home and then that is the child's 'share' of the bills) so that the child does not have to find a job to go to rather than school.
Tell the child who is having a fit over this that she isn't an adult and doesn't make the rules and the rules say it is for household expenses and NOT for her to spend as she wants. Because it is NOT coming to her and she cannot spend it how she likes. As the PARENT it is up to YOU to spend it on her behalf and those ARE the rules. I helped one of J's friends understand this when she was a young teen. Her gma and gpa were retired and raising her and her bro and sis and she couldn't grasp why she didn't get the $$ to spend as she lked. She has Fetal Alcohol Effects (FAE) and that swiss cheese memory thing and it is truly very hard for her to remember things like this and it is hard to live in "groundhog day' to use the movie reference. So she asked me because her Gma was getting very upset because the girl was having tantrums over this money. I sat down and showed her the things it was to pay for - and how much it was likely that her folks were paying because their home was bigger and less energy efficient than ours so I used our bills. By not being her parents, she was not angry while asking the questions and she was able to listen, understand and remember what I said more, so ti solved some HUGE issues for her parents during an overnight at our house. This girl does not sleep (mom is an addict and used every drug she could esp meth and booze while preg and it cause severe issues for her bright, beautiful daughter - most people see the daughter as stupid but she is NOT, she just cannot use it in typical ways because the Fetal Alcohol Effects (FAE)) so during sleepovers I stayed up all night. Otherwise she got bored and it could be difficult for her in a strange house. Plus I LIKE the girl and enjoy her, so we would use that time to talk alone. She took a lot of problems to me then, because other than her gparents, she didn't have a lot of people she could talk to. We bonded, and she hasn't forgotten it.
Explaining that the money is so that she does not have to get a full time job to pay for the rent and utilities may make this go a lot easier because it will be memorable and ti sort of moves things out of 'kid' responsibilities. Esp if you make the difficult child believe that if she takes this $ from you by stealing from a purse or wallet or taking the bank card info, then she has to get a job to pay her 'share' of the household bills. Kids do NOT think they need to pay those, so making her think that she will have to can be a shock that is memorable.