R.M.
Hi, I believe that childcare is like any other profession and an employee should have a performance review (type meeting) with discussion of expectations of the upcoming year, the benefits and yes, salary. If you really like this person and want to continue with her it would be best to be up front with her and not "wait until she brings it up". If she gets the point that she feels she has to bring it up she may feel unvalued and it could cost you more in the end. Maybe extra pay isn't the issue for her..maybe one more day of vacation/sick time or other benefit would be something you could discuss.
If you feel that the 1/2 to 1 hour you come home early often is a benefit to her, you might want to see how she views it. If she is perfectly happy to stay for her scheduled time but you tell her to go home early, she may not view that as a benefit like you do. I think that if my boss lets me leave a bit earlier than I planned, it's nice, but I don't look at it as "extra" money.
I hope this helps. Good luck!!