Work frustrations (rant?)

I'm a nanny who works 50+ hours a week with two awesome little kids.  Normally, I think I do a pretty great job - we do lots of learning activities, fun field trips, and have lots of active play.  Then when they nap, I normally do laundry and dishes and tidy the (enormous) house. 

But, of course, I'm pregnant right now and SUPER tired (plus hungry, plus nauseous, etc).  I don't feel like myself - I can't do anything I would normally do because I just don't have the energy.  I find myself doing a lot of my chores while the kids are awake just so that I am guaranteed an hour or so to rest while the children are asleep.  I know I am slacking off compared to my normal workload, and if the parents knew, I think they might be appalled!  (Obviously they are already aware of the fact that I'm getting behind on the laundry, but they don't seem bothered by that at least - if they did, they'd say.  They are very particular.)

So anyway, I guess this was mainly just a rant about how exhausted I am and how much I wish I had a more sedentary job.  I could probably muster the energy to sit at a computer and crunch numbers all day (like at a job I had a few years back), but I feel incapable of chasing after a 3-year-old and a 1-year-old.  

Any advice?  Don't say 'temporary leave of absence' as it is out of the question.  Just wondering if anybody has any ideas how to cope.  I also wouldn't object to being reassured that I'm not a terrible nanny for taking it easy for a while...   I have a newfound respect for all those women who are pregnant with #2 or higher... How do you do it all?!

Alright, I'm done whining.  Carry on!