Have it checked. It is probably just plaque but better to have the opinion of a pro and dental work on kids is not very expensive. Turns out my son (5 yo) has no enamel on his two bottom teeth coming in (his adult ones) and the dentist has been great in talking us through what he'll need.
One word of warning, a pediatric dentist is not necessarily the only route to go. I started with a pediatric dentist for my son that I picked through my insurance. It was a HORRIBLE experience (they were rude, impatient, didn't make allowances for my son's social-emotional needs, physically forced procedures on him, tried to push having him sedated, and then sent me a letter threatening to report me to CPS when I refused to bring my child back to them for more treatment). Then I switched to a general dentist that was recommended by a friend whose kids loved this dentist. What a difference! He doesn't specialize in kids but had a great manner that was so perfect for my son and his sensory issues. I just pay out of pocket because he doesn't take the insurance I used to have and now I have no dental insurance.