I suggest to feed the child before she arrives too and then you can say its fine he is fed
maybe leave some of lunch left over for him so you are not relying on if she brings his supper or not
or even crackers and cheese so he can have something to eat with her later
start the hard talk with ,,,,,, you don't mind if at the end of the day he is ready to go as I need to feed my family or have family time
good luck