You should be fine in the Suite side. You and DH in the King Master, the 2 older children in the Murphy beds and the baby in the crib. I would imagine that they will not even charge you the activity fee for the baby, especially since it is only one night.
We paid $25 per night for an additional teen last April at the Haciendas. In recent years it was $20 per night but has gone up. Well worth it, she had a ball and would have had to stay home since it was her older sister's senior trip.