I would start by being honest with your mother and let her know that you do not trust her. Tell her that she is no longer welcome in your home because she is putting her grandchildren in danger. Than cut ties with her for a few months. I know this sounds harsh and evil of me to say but right now she IS putting YOUR children in danger.
Than make sure you have all of the legal paperwork you need to show CPS that your children are within the law homeschooling. There is no need to speak of anything more about your right to home school. Sometimes the school officer just needs to be informed that you are legal. Paperwork gets crossed some times.
Do not let them in. PERIOD. They will try EVERYTHING to get in. They will tell you "I need to see your cupboards" "I need to see where the children sleep". ANYTHING. Respond with, Most currently as soon as you get a court order.
And most important thing ever. DO NOT show that you are upset. Any outburst shown by you will only give them more reason. Sing a song in your head, count to 100, do whatever it takes to stay calm.
Work on how your posture is and make sure you are not standing with your hands crossed. Practice in the mirror if you must. Be overnice, but not accommodating. And breath, once they leave you can break down and cry if you must but do not get on the phone and call anyone, or post on social media that you are upset.
Stay strong and you will get through this.