I'm going to tell my friends that something they made fun of me about was very sensitive to me
What did your friends say to you that was sensitive to you?
They make fun of me for being childish.
If you feel you're hurt calling you childish, you definitely should tell your friends that it's sensitive to you. I hope they'll understand it and will stop making fun of you.
I was being sarcastic, why would you even believe me?
Well, if they ever do call you names, you should still stand up for yourself. Friends shouldn't make fun of their friends for things they feel sensitive about.