Stress
Every job has stress. What kind of stress are you willing to endure? Physical? Mental? Emotional? Well, good - that's what you'll be getting.
Working as a preschool teacher is a lot like being a parent. From the outside, it looks like a lot of thankless, barf-covered work. From the inside, it's worth it. Just keep in mind: Children are cute and funny, and you only have to work ten months out of the year.
You'll need to raise children according to principle and a few laws and regulations. There's paperwork, reports, education, workshops, communication with parents, faculty, doctors, specialists, and humans small enough to head butt you in the knees.
You have to be careful that sanitation procedures are followed and nutritional needs are met. You have to be sensitive to cultural differences. You have to work with playground equipment, toys, and cleaning supplies. You might have to raise funds, set up vaccinations, and handle emergencies.
Oh...and you'll be tired. But no napping when the lights are turned off and the kids are quietly sleeping on their cots, the afternoon light gently filtering into the peaceful room, little snores and hiccups, lulling you into—oh, no you don't. You can sleep later. Drink some Red Bull.