I agree that this is a difficult situation to be in, however remind yourself that she would not accept late payment from her employer, so you should not have too either. The longer you wait to address this the more difficult it will become so either call or text as soon as possible to give a gentle reminder.

I understand that when the kids are sick and they are not coming over, it makes it more complicated and they might actually forget. However, I believe that there is no valid excuse as you deserve to be paid on time, no matter what the situation. Especially since she has several options, such as stopping by on the way to the doctors, send hubby when he gets home and/or do the online transfer....

Just keep telling yourself that in any other job you would have your payment on time no matter what and this is no different.