Let me get this straight. She regularly collects your children from school and walks with them to them to swimming .While they are there she must hang around waiting for them to finish, then she supervises them getting dressed and ready to come home. You pick them all up and drive them home to yours, where she comes in and have a chat (and maybe a cup of tea) and gets a bus or walks home. Last time, you didn't ask her in, but put her out at your house without saying why. Now she thinks YOU are annoyed with HER?
Why is SHE not annoyed with YOU for treating her like dirt?
Paid or not has nothing to do with it. If you pay her, she is not doing this as a favour, but it seems to me that paying her has given you the feeling that you are the big boss and you can change things suddenly without any warning. Of course she is hurt. She feels she wasn't worth an explanation of WHY she wasn't welcome.