I give my 17mnth old 1/2 teaspoon of children's zyrtec (cetirizine) for allergies but some days it's not enough. Can I increase the dose or try something differ t?

Do not increase . A 17 month old should not have allergies to inhaled items like dust or pollen. Zyrtec (cetirizine) could cause drowsiness and i would avoid long term use of zyrtec (cetirizine) in your child. If your child has a runny or stuffy nose that is unresponsive to zyrtec (cetirizine) then see your doctor to make sure there isn't an infection brewing.