I'm a little confused. You paid $400 for 4 weeks? You are paying $100/week for childcare? If so, then I think I see part of the problem but I can't be certain in case I'm wrong about that.

I would never cancel a contract if clients were paying full fees for a leave of absence, no that would be unfair. However, if the contract states that I or the clients can provide 2 weeks notice and terminate then that is allowed. It depends on how the contract is worded. I suspect this caregiver interviewed other families while you were on your sick leave with her and she filled your space and is now giving you notice. Maybe she was worried you wouldn't come back at all and decided that she better fill the space? I'm not sure.

Maybe it feels unfair but it could be possible that this caregiver is within her rights.