Usually if a overnight CM or nanny is used they have contact details of next of kin if there is an accident. If the OP MIL is holidaying out of the country or would be unwilling to cut her holiday short then this is outside of the normal arrangement.
By law you cannot give a stranger 'proxy parental responsibility', there is normally one member of the family who can be contacted. If there is a family member who will oversee the arrangements then that would be fine. If the only other person who has any 'responsibility' for the children is in hospital then this is not ideal. These sort of arrangements are usually built up over time. What a lot of posters are suggesting (without realising it) is that the OP enters into a private fostering arrangement (a very dodgy area).
The OP will need recovery time, she may have a longer hospital stay and needs a proper plan in place. As someone who works for SS i would urge you to conntact SS and at least discuss your needs.
OP- They will make a plan that suits you using approved care. You will know where your children are and what they will be doing. Your children will be returned to you. This situation is quite common. As i said the SW will also give you after care.