a body is a body is a body. Doesn't matter the age or size. infants matter in the occupancy count, so do 2 year olds. The only benefit (if they are #3 or 4 in the room) is the rate for them is so little.
So yes, you will need 2 rooms for 5 people unless you opt for a cat 4 family stateroom (or higher), which holds 5. The advantage to 2 rooms (consider connecting rooms) is that you get 2 (sets of)bathrooms.