I'm going to let my son use my daughter's ball
Why would you let him use your daughter's ball?
Because I'm a great dad and he would be grateful for that.
As long as your daughter is okay with then it's good for siblings to lend things to each other. Just taking the ball away would lead to resentment from your daughter.
She will be okay with it.
That is good. It shows you and your children have a great relationship with each other.