Hi J.,
Preschool asks a lot of children, but there are usually good guidelines in place for kids so that it's not too much. You may experience some tiredness and regressive behaviors and that can be attributed to the fact that they are learning so much in some areas, others become less important to them. Toileting is a common one--too busy at school to stop and use the bathroom. What I want to point out is that, if you see regression and tiredness, don't chalk it up to their being unhappy at preschool. They're just getting used to this (and don't they get tired and regressive at home sometimes anyways!) .
One other thing I'd suggest is making sure your little ones are getting a good chunk of time alone after school on your short days. Try to get shopping, errands done before picking them up, and plan on their being emotionally unpredictable. I advise families at my preschool to give the children plenty of downtime after school. This could be a short nap, books on the bed sort of 'quiet time' or unstructured playtime, where each child can play alone in their own space for a while. There's a lot of social work that's required of being in the group at preschool, so this time can make one's afternoon much better than heading out to another activity/playdate, etc. Earlier bedtimes can help the morning routine, too, if you are having trouble getting out the door. Lots of rest in general is good for kids this age; they need anywhere from 9-12 hours of sleep.
Best wishes on your family's plans. For what it's worth, I stopped nannying about 4 months into my pregnancy, so I understand just how tiring it is. Don't feel guilty and get some rest!
Best,
H.