My DH is pretty much self employed so it's very rare that he can't work it out to stay home when DS is sick. Sometimes if he has a meeting he'll drop DS at my office for an hour or two. I work alone in a sattelite office so as long as I don't have clients in the office I can bring him with me occasionally. So here's our emergency plan in order:
1) DH keeps DS and works from home, dropping him at my office for a few hours if necessary for meeting, etc.
2) MIL/FIL. FIL is retired but works part time supervising the writing of standardized tests for Social Workers and travels about 2 times a month to meet with the team he supervises.
3) He comes to work with me
4) I ask to work from home
We try to have an established game plan at the beginning of each winter and it tends to make last minute illnesses a little bit less stressful for me since DH is worthless when it comes to handling most situations that require decisions to be made