New clothes have enough chemicals in them to kill a horse. Babies and children are very sensitive to them and consequently all new clothes should be run through the wash at least once before wearing.
Another alternative is to buy your clothing second hand. My mother-in-law has always bought our kids second hand clothing. At first I thought she was just cheap, but then I realized that she was absolutely right after I saw the results of chemical clothing on my kids' skin (Itchy red rashes from wearing the new, unwashed clothing.) Usually a good wash or two would render the garmet harmless.
In the US, all children's sleepwear has to be flame retardant which is accomplished by soaking them in chemicals, so it is best to avoid them all together and opt for the leggings/underwear route. If you are concerned about your kids spontaneously combusting, don't let them smoke in bed.