Be careful with different cultures, as "tonto" and "tonta," and even "tontito/tontita" can be offensive, meaning "stupid." If you are ever unsure, use the word "juguetón" o "juguetona" which refers to being silly in a playful way.
Thanks for your interest in our dictionaries. I will transfer this report to the team that manages the English base dictionary so they consider adding a nuance or example sentence that can be translated as tontito and similar things in Spanish (for example, when someone feels embarrassed or worried about something and you reply Don't be silly!)
Thank you for your message. @n.ares, I've actually updated the sentences for all the senses of silly fairly recently (in response to a report from enfr, if I remember rightly), so wait and see when those come through. Hopefully they will make it clearer. But feel free to come back to me if they they don't.