My personal view is that your boss has something to say

behind his mind. If he really did not care that you were late, he would say 'That is fine, glad you were not in it...bla bla."

Being punctual is hard for you who has a young child. But you'd better try your best to be punctual. Being puctual is the attitude for your job. No matter you do a good job or no, if you are always late, that will make a bad image in front of others, and your boss. Also do not expect others will be tolerant for your being late, since it is not fair for them who are not late. It is also hard for your boss to require others to be in time if he can be tolerant for your case.