I think there are separate issues.
First, it is legal to furlough someone for childcare responsibilities and so it may well be that those people couldn't have come to work and it's potentially cheaper for an employer to furlough than pay full sick pay depending on the policy (if their GPs had signed them off sick).
Secondly, if cancer doesn't automatically make you clinically extremely vulnerable, especially if it was removed by surgery 6 months ago and then you've had no further treatment which would make you immunocompromised. If you had chemotherapy or other treatment which affects your immune system, then you would be in the shielding category and your employer would be expected to either pay, furlough or allow you to work from home. This should be reflected in your risk assessment and have clear rationale and information about what is in place to keep you safe.
If however you are anxious about catching covid, but don't fall into the shielding group, then it's harsh, but your manager doesn't have to furlough you because of anxiety about catching covid. In that case, you would need to be signed off sick by your GP due to anxiety rather than because of your covid risk. The GP also may be able to justify putting you in a shielding category as they have access to your medical records, even if you are not in the official batch from the department of health - s/he will be able to write a letter for this if needed and confirm you should be shielding if you've been missed from the list.
It's not about putting mothers above someone with cancer, it's about looking at individual circumstances and taking them in to account, when work still needs to be done.