The perfect accessory for nebulizing is the mouthpiece, because it lets you inhale the medication without the risk of losing any pieces. It's like a superhighway direct from the nebulizer to your airways, but do keep in mind that it isn't the easiest method to use, as you have to keep your mouth shut and hold your nose closed with your fingers. It certainly isn't the best type of equipment for your child because it involves forcing these actions on him, which wouldn't be easy for him to get used to.
If your child needs treatment with a nebulizer, the accessory we suggest you consider is the face mask. You just have to keep in mind that its main disadvantage is the loss of medication if it isn't positioned tightly enough on the face.
Here's the nosepiece, another accessory you can choose to use for inhaling medication with a nebulizer. Remember that it's an accessory that's only for use when you need to nebulize medication or saline solution onto the mucous lining of the nose for keeping it clean. There aren't many available of this sort on the market, as most people prefer the nasal wash.
The nasal wash is the best alternative to the nosepiece for nebulizing with saline solution, as it's less finicky and much quicker to do. Keep in mind though that it's only done with a piston nebulizer when a suitable amount of pressure can be employed.