S.T.
Don't be in such a hurry to switch them to one nap. You know your children better than their doctor. My son is 18 months old, and he still takes two naps. He literally can not make it until the afternoon without a nap. I've tried a few times, and it doesn't seem fair to keep him up when he is basically begging to go to sleep. I figure he'll give me clues when he's ready. My son goes to bed around 7 pm and wakes between 6-7 am. He takes his first nap from 9:30-10:30. His second nap is usually from 2:30-3:30. I hope this helps.