Well, you should not have allowed her to be exposed to gambling at such a young age even though there's a chance that she might end up discovering it by herself when she's older but this is better because she would have the mental capacity to handle the pressure that comes with gambling. You can't change what has already happened but you can start guiding her so she won't make mistakes as she gets older
It's always my opinion that parents look should do everything they can to look after their children and prevent them from getting involved in any thing or activity that they are not yet qualified to participate in as children who are underaged.
Underage simply means that the child's brain or mental faculty isn't yet well developed enough to handle the who stress that goes with gambling, but in the case of the op, the deed has already been done and only thing he can do now is just (exactly as you have said) start guiding the child and talking to her always about the dangers of excessive gambling or gambling too much.