I will call if: the child is unable to participate in our activities, there are other symptoms of illness, the child seems distressed or uncomfortable, or I feel that the situation should not be managed by me anymore. If the kid is blowing through a diaper an hour, then I will give mom and dad a call, and at least give them the option of picking up Jr.
If it's just icky, that could be caused by so many other things in toddlers and babies, so I don't worry about it.