The 2-1 nap transition can be easy breezy or painfully challenging. It can happen as early as 12 months (that is very rare), or as late as 18/19 months. A child will typically be ready for the switch to just one nap between 15-18 months.
There are 2 ways that a child’s morning nap can disappear. The first way is the way that we all hope our children drop to one nap. The child just stops falling asleep for their first nap at an age appropriate time (9:30/10am). If this occurs for 2 weeks, you simply stop offering the morning nap using natural light to keep them awake, and begin offering just one nap at 11:30am. After doing an 11:30am nap for a few days you would then move it to 11:45am for a couple of days. After that you move it up to 12pm. That one nap a day typically starts between 12pm-12:15pm, getting later as the child gets older.
The second way a nap can fade is more common. What happens is that the sleep drive is SO strong at 9:00/9:30am, that the child continues taking their first nap but then doesn’t fall asleep for the second nap at an appropriate time (by 1:30pm/2pm). In this circumstance a parent or caregiver will want to start addressing this change by “capping” (waking the child) the morning nap to be 15-minutes shorter than it has been. If that reestablishes the second nap, stick with this plan. If the second nap still isn’t reestablished, cap the first nap another 15-minutes
If need be, it is okay to continue to cap the first nap to reestablish the second nap until it is only 1 hour long. If the child is still boycotting the second nap after their morning nap is just 1 hour (sleeping from 10am-11am latest), it’s time to drop the morning nap and push for one nap using the plan described above.(11:30am for 2 days, then 11:45am, then a noon nap). Most children who are around 15-18 months will start their one nap a by 12pm/12:15pm.