Hi Elizabeth!
Our children are close in age, as my son is 3 1/2 and daughter is about to turn 7. Are we having fun at this or what?!?! I'm realizing now that Disney World is, in some ways, my therapy :-)
It depends on if your plan was to go to a park every day, or have one day off in what sounds like a 3-day visit. I'd say this: if you're staying 3 entire days, get the 3-day pass. It always takes my family 2 days to really experience Magic Kingdom. Not just from the attractions, but shows, parades, character experiences, dining at this iconic park can stretch over 2 days easy.
If you're only looking at Magic Kingdom and Hollywood Studios, I would just get a 3-day base ticket and not get the Park Hopper option.
May you and your family have a "Disney Blast!"