Feel really sorry for your kids. If I were you,

I will move out immediately and rent another apartment. Write the landlord a formal letter telling that you are moving out. You really should talk to a lawyer. A letter from a lawyer can be very helpful. At the same time, take pictures of what the apartment was like during reparation. Take the pictures of your kids falling. Your kids' health and safety is No1 priority.

Tell you 2 cases: my friend's apartment's kitchen was burnt (partially) during a leakage fix. The management move her to another apartment without any issue. The 2nd case is also in CA. Another friend wanted to break the lease because there were ants in the apartment and she felt it was not healthy for her kids. The landlord agreed.

I believe your landloard will be able to find another tenant after the condo is repaired. So, for the worst case that it was judged you should not break the lease, you won't be liable to $21000.

Act now!