Sneezing can mean a few different things.
1. too dusty where they are being kept, either at night or during the day.
2. Something irritating their sinuses like smoke or something else in the air.
3. First signs of infection. What is called a "cold" is an infection that is either fungal, bacterial, or Viral. The first 2 can be treated, but Viral is probematic. With a virus the symptoms can be treated, but tend to come back when the bird is stressed and spreads even when no one is symptomatic.
I hope it is one of the first 2. If you have them in a brooder at night make sure to have good ventilation. I would change the bedding now. If you use shavings, make sure they have no cedar in them. Wipe the brooder down with a damp clothe to cut down on the dust. I use a very mild bleach solution to do this.
Be sure there are no strong scents near them. I had a friend that was buring a scented candle in the same room as the brooder and got all her chicks sneezing.
As long as they don't show any other symptoms and the sneezing clears up in a couple days, I wouldn't worry.
Matt