Naps.. how long are they really needed?

Naptime here at our house is very important,  if it is missed then the nighttime sleeping is not so good.  I remember thinking tire her out and then she will sleep a lot longer and better.  Wrong!  For some reason the more tired she is the worse she sleeps.  That does not make a lot of sense to me but that is how it works with Ashlee. 
Sawyer on the other hand seems to sleep well whenever!  I am not sure when Ashlee does not need her afternoon naps anymore.. lots of people have kids her age and they no longer have naps, but I have tried and she still wakes up at night if we skip it.  Maybe I should just shorten her nap from 2 hours to 1 hour.? wonder if that will help?
Ashlee is going to be starting 4 days a week of afternoon school in September and I need to get some ideas on naptimes.  Would an hour nap after school work? 245 to 345? or there about?  When there was just one child it was easier to move naptime around, with two, one is still very relient on that schedule.... help? does anyone have suggestions?
Follow up; I tried letting Ashlee go without the nap in the afternoon and she told me after 4-5 days that she was just not ready for it.  She was tired all the time.  We worked out a solution of her taking at least an hour nap for now.  Her additude has improved about it since it was her idea.