Low temp combined with complaints of pain or discomfort or a lightly unhappy stomach, yes I'd keep them home. I don't like being seen as the parent who sent a sick kid to school.
I do have to agree with the PP who said that some kids do not come across as sick. It can be a hard call sometimes. I have a perfect case in point.
When my son was 1, he had what I thought was another of countless ear infections. No fever just cranky and sleepless. After about three days of this, he woke up exhaling hard on every breath. His temp was normal, but he was very lethargic. I took him in and he was diagnosed with pnuemonia, and was in the hospital for 3 days, then sent home.
18 hours later he had a fever of 101, but was acting fine.

I called the ER because even though he was happy and playful, I didn't think he should have a fever again when he was still on all kinds of meds. They said bring him in and they would check him. I took him to the ER and he was toddling around the waiting room, playing, flirting with other people who were waiting. He was acting totally normal and I felt like we were completely wasting our time.
They checked his blood and his white counts were off the charts, worse than when he was first admitted with pnuemonia. They readmitted him and we ended up spending 18 more days in the hospital with a nasty blood infection. (He didn't start "acting sick" until later that evening.)
So I can see how sometimes this is a very hard call to make and I try not to judge parents harshly when they make a bad call. (Oh, and I'm a day care provider, so I do totally get how annoying it is when a kid is given tylenol and sent in for the day. Been there, done that. Just had to play devil's advocate and say that occasionally it may not be the case.

)