Ok, I'm going to admit that after reading for awhile I started to just scan, because your post reads pretty much like dozens (and I do mean dozens) of others who have young children and are considering nursing school
or going to nursing school now. I don't mean to offend, but your situation is far from unique.
Many MANY people juggle young children/older children while going to school, myself included. Frankly, because so many people think of nursing as a later-start career (because they were laid off from another industry, because their first degree is useless, etc etc) it's as common as dirt.
The bottom line is this: what do you want to do, will it improve your family/home life by doing it (at least in the long run)? If it means going to school, then you WILL FIGURE IT OUT.
What do you do when one of the kids is sick? Better have a backup plan, because you're right: you will NOT be missing classes and/or clinicals because of it. School vacations, holidays, snow days....always have a strong backup, because you WILL NEED IT.
Sometimes we know what our kids need medically ahead of time. Sometimes not. You know your husband will not be available much; do you have a strong network to pull from? If yes, school is an option. If no....it really isn't.
If you are dedicated to the desired outcome (becoming a nurse), then the rest is just scheduling. Guilt? If you're an unhappy woman who will always regret not returning to school, believe me this won't impress your kids later on down the road. A happy mother is a better one, IMHO.
Now, if you're the type who will put school on the backburner because of glitches (snow days, sick days, etc etc etc), it won't matter that you "want to help people"...you won't succeed in school.
Decide what your goal is, and GO for it.