I need a backup plan (RE: childcare)

When the twins arrive, my mom and MIL will be dividing the week to come over and watch the babies once I go back to work.

For whatever reason, it never occurred to me that there may very well be a day that neither mom is available (illness, out of town, appointments, etc).  So now I'm sort of panicking that I do not have any kind of backup besides me or DH taking off work.  For me, I need to save all sick days if possible.  DH's work is more flexible, but he can't just take off all the time either.

I don't even know where to begin looking for a plan!  Any suggestions?  I'd prefer it if I could have someone come here, but I'd be willing to take them somewhere as well.

Thanks!