I personally am a firm believer of breaks but only when the parks are open past 7-8pm. I am not so sure the hours in March but that would be a BIG deciding factor for me. As much as I love breaks, they take some time. If you are in Toontown, it will take you a good 15min just to get to the front gate, then wait for the monorail, back to TTC, wait for the tram, get in the car and then back to timeshare, then settle down or change into swimsuits etc... - you get the idea. By the time you wrap up the break and head back it will probably be around 4-5pm and if the parks close early then it is only a few hours. The fact that you are offsite makes it even longer.
I would probably go back and forth. Maybe do a real early morning on a day the park is open later like Sat. Take a break that day while it is really busy like 1-4pm and head back for the night. Then maybe go all day straight the next day but then sleep in a little later the next day and so forth. Play around your kids - you don't want them to crash at night because some of the best stuff is at night and cranky kids aren't fun. If you need a break, take them. I would also recommend sit down table meal lunches or late sit down breakfasts and then counter service dinners as opposed to early character breakfasts, counter lunches and table service dinners. Why? No one is THAT hungry first thing in the morning - since you have a timeshare, I would eat small breakfasts before leaving. Arrive early and hit the rides and go commando while it is not that hot and not busy - places like Tomorrowland and Fantasyland have lots of rides in a little bit of space - hit them early. Then take a break in a nice AC restaurant and rejuvinate while someone serves you - lunches are never usually that busy and are cheaper then dinner menus anyway. Then while people are switching parks, eating dinner etc... you can head on some rides then maybe catch a quick bite to eat late and then enjoy the nighttime festivites at whatever park. This may help keep you there all day. Also the weather isn't so hot in March so that will help ALOT. Changing parks (if you have hoppers) always seems to rejuvinate us too. A nice A/C monorail ride from Epoct to MK and vice versa is nice!
HTH and have fun. Remember every kid is different, listen to what your gut is telling when you get there. You won't know before then how they will handle it anyway. They may run all day or be sleepy by noon. No kid is the same at WDW as he is at home. Just wait until you are there to decide and leave your options open, don't plan an exact itinerary or you will be disappointed.