Hi
I would stay onsite, no car rental fee, no Theme Park parking fee, No Hotel Parking Fee and no airport Arrival / Departure shuttle fee.
Spring/March Break is busy and it is also one of the more expensive times to visit.
Just a note almost all of March and part of April are various Spring Break/March Break for many visitors.
As the previous poster said utilizing EMH (Early Magic Hours) with two young children might help your touring style,however I am not sure getting two little ones up early is going to be easy.
I would only get 4 day passes and take one day break in between to pool relax and maybe a visit to Downtown Disney /
maybe
bowling
dine in Movie theatre
I do know the one thing my grandaughter has wanted from our past trips is more resort time.
I swear she is a mermaid.
Epcot is great during this time as it has a few more activities for kids to do ,playgrounds butterfly exhibit etc. (Epcot Flower And Garden Festival).
Note that March 17 St Patrick's day has a few more activities going on in Downtown Disney (DTD) soon to be called Disney Springs.
Price wise
6 night five day stay at Pop Century is
March 15-21 2015.
Walt Disney World site quote
4-Day Theme Park Ticket
2 x Ages 10+
1 x Ages 3-9
US$1,934.71USD
You would still need to budget for food and souvenirs,however a quick trip to the dollar store ahead of time to collect and make a few treats for the kids will save you alot on souvenirs.
Have a great time planning
Hugs Mel