Is it ok to give a decongestant to a 2yr old?

No. Decongestants are vasoconstrictors. They get absorbed from the nose to the bloodstream causing rise in the pulse and blood pressure and vasoconstriction to the coronaries and the brain with possibility of stroke, heart attacks, cardiac arrhyhmias. If desperate because child cannot breath through the nose use nasal saline or talk to the pediatrician to decide. After age two most medicines are allowed.
Should not. Nasal saline, apply with q tip. Vaporizer in bedroom at night during winter to avoid dry air to irritate airways.