Try 'Yellow Lotion' on it, it is a Ranvet product and I have used it on several wounds that looked much much nastier than this one with zero proud flesh after they were healed. This wound looks like it is at the perfect stage to use it so don't worry yet! All you do is bandage the wound twice a day with some yellow lotion, cotton pad, gauze etc and you won't have to worry about proud flesh at all. It is also an antibacterial so will prevent infection.
All horses deserve, at least once in their lives, to be loved by a little girl.