She probably will be tired for the first few weeks until she adjusts. But it's nothing she can't handle, I'll bet. If you find her groggy in the mornings, exhausted (or wired!) at night, or irritable, you just move her bedtime up a bit.
(A teeny tangent: When I put my son in school (I homeschooled the first few years, so he was nine), the teacher called me after about three weeks to tell me that Adam was falling asleep at his desk in the afternoons. It hadn't occured to me to change his bedtime, even though, because of school, he was getting up an hour earlier every day! A little embarrassing? Um, yeah - but easy to fix!)
If she's been mixing it up with other kids a few times a week already, you may not find she gets markedly more bugs and viruses, but a backup is a good idea, just for your peace of mind. Is there someone in the neighbourhood you could approach to be your backup in such a situation? Or perhaps someone who specializes in sick-child care? (They're generally expensive, but good to have on call.) Is there a college in your city that offers an ECE or Child Studies program? Maybe you could contract with a student to be your backup.