Hi Cara!
You are traveling during a relatively quiet time for Walt Disney World, with the exception of the weekend of March 6th and 7th which is the Princess Half-Marathon Weekend. However, in my opinion, even on that weekend the parks shouldn't be that crowded.
Your plan of visiting the parks in the morning, taking a break in the afternoon, and returning in the evening is my usual recommendation. I especially recommend that during the summer when the parks are generally hot and crowded. However, since you are visiting the 1st week of March, you may not need to do that. An alternative plan, that may work out better for your 2 year old (who probably goes to sleep early at night) may be to stay in a park the whole day and then you could just leave around dinner time; of course, if your 2 year old is anything like my kids were when they were that age, he or she will probably nap in the parks. I suggest the alternative plan, because your older children may not want to leave the parks.
However, Stormalong Bay, the main pool at the Beach Club, is outstanding! Your children may want to spend as much time there as possible. That includes your 2 year old, as the children pool there has a sand island in the middle of it; it's a great area to make sand castles!
I recommend that you arrive at a theme park at least 30 minutes before the posted opening time. That's also true if the park is having morning Extra Magic Hour (EMH). I personally love morning EMH as I think a lot of guests like to sleep in. However, I think it is also true that a park offering morning EMH is generally more crowded later in the day than it would be if it wasn't offering EMH. You can my thoughts on the subject
here.
Write back if you have any more questions!