Jaffa cakes are British sponge cakes with orange jelly and chocolate, made by McVitie's. Subject of famous tax-classification cases about whether they are cakes or biscuits. Not concerning, just food. If your kid uses it constantly, ask them where they first heard it, not to police their speech but to understand which online community they are in. Most slang of this kind moves through TikTok and group chats, so seeing it in your child's vocabulary just means they are plugged into normal teen internet culture.