Would you speak up in this case to boss?

The woman I provide part time child care for is kind and flexible on the whole.. her son is so cute and so smart and I do enjoy teaching and caring for him. I am someone who is a bit shy about speaking up though; I often just let things 'go' for the sake of being agreeable. I am unsure whether I do have the right with only part time shifts to say that I need a set schedule; I never know what time I'm getting ou t.  IN the beginning she told me sometimes, she'd be late, running just a half hour late than normal. But now it is very typical for once a week that she's running late, and some days, she could arrive early , or again, late. I don't want to seem ungrateful for the position, and want to be 'available' like I said I would be if she was late, but would you say, 'It would be beneficial for me to know what time I'm leaving' ? I have had to cancel two things already b/c she was late. another thing I'd like to bring up to her is how she overrides what I say to the child , in the case of running.. he is only a toddler and prone to falling here or there. So i tell him not to run, and then she'll say, 'mom says you can run', right in front of me... after we had already talked about why I ask him not to run. I am unsure how to use my voice, or not.... any tips welcome...