If you stay in the park you can get early entrance and a fast pass which makes it doable in 1 day.

I have searched the archives but not seen any commentary on crowd size the first week in March.

There are several very informative threads on this in this forum. Do a search. I don't really want to retype everything.

I think I would actually hit two of them first thing. Probably hogwarts then take the train over to the other park and try to take gringotts before it gets too crowded. Then you can walk back to the first park (about 15 minutes) to do the other things at that park. The problem is that most people try to spend half a day at each park, which means that the lines for the hogwarts train in mid-day are absolutely horrible. Its not really efficient to do so much back and forth, but it's quicker than standing in line for an hour mid-day in order to switch parks. There's basically no line for the train early in the day.

If you want to see both of the Harry Potter themed lands, you will definitely need the park to park ticket. Hogwarts is at Islands of Adventure and London/Diagon Alley is at Universal Studios. In order to take the train back and forth between the two, you'll need to have a valid theme park ticket for both.

The theming very well done. Yes, it will likely be packed (especially the newer London area) -- but if you are able and willing to go first thing in the morning, you should be able to see a decent amount before it gets super crowded around lunchtime. The major attractions (Forbidden Journey, Gringotts, and the train ride) are the busiest, so it's a good strategy to hit one of them first thing and then take your time browsing the shops and scenery.

If you're just planning to hit HP attractions, don't bother to get the Express Pass -- it's not valid for the major rides at each park.

You can bring bottled water and small snacks into the park. Unlike WDW, you can't pack a lunch and bring it in. Check the Universal website for more details.

We will be in the Orlando area the first week in March. Considering a one day swig thru Harry Potter world for out three HP-obsessed children.
Is it worth it?
Will it be packed?
Can we bring snacks and drinks in?
Do we need the park to park ticket or will Hogwarts alone suffice?
Kids are 5, 7, and 11. They handled WDW well a year ago, so I'm not worried about stamina, etc.