I don't know if they chew. I haven't seen them since they were babies cause they live on the East coast, & I'm on the West one. It has been a long time since I knit for kids. Other that baby layettes, which don't use buttons anyway. So I'm pretty clueless about what age is okay for buttons.
The majority of 3 yr olds are beyond chewing on things. That's usually done by kids around a year to maybe 2yrs. Three year olds have more attention span and skills now that keep them busy plus they are at the age you can actually talk to them. Personally I wouldn't worry about it, but go with what you feel comfortable with.
When asking questions ALWAYS post the name and a link for the pattern if you have it.