to keep a promise

When she wants a candy or an ice-cream, she will say she will brush her teeth afterwards.

However, there have been occasions that she runs away and refuses to brush her teeth at night time.

Yesterday I talked to her seriously about such and asked her to keep her promise. (Michelle Yeeyee said she would not be trusted if she fails to do so.) She said she would and then I gave her a tiny chocolate bean with strawberry filling. I reminded her to keep her promise. Then she asked me innocently what was "to keep a promise" after the chocolate bean melted in her mouth.