OP give them a telephone call and explain your situation. Most areas are very understanding currently, we're all having to make some unusual choices but if they've gone to all the trouble of recruiting you right up to a start date of Monday they'll want to try and keep you
Im new to start a new job on Monday, It is a key worker job in the health care sector, My DH is also a key work who can not take time off as his job is needed more than ever with very few qualified in his field. My issue is now childcare, Our plan was ment to be GPs helping with childcare but this is no longer applicable as both sets of GPs are high risks which we will not take a chance on. Our DC are 3 and 8months. Im going to call into day and ask for a 3 week delay. Is this unreasonable as I feel so guilty and just hope the dont retract the job offer.