Do you move up bedtime? Do you wait it out and see if they sleep longer?
DS usually sleeps from 12-2...but some days he'll sleep 1 hour - 1.5 hours.
We try to wait it out at least 10 minutes, but if he is standing/sitting up for a good five minutes and starts whining, we just end the nap.
We'll try to move up bedtime a lil bit and keep him up instead of another cat nap later in the day.