How do I shrink a pre-shrunk T-shirt with a decal on it?

I have this T-shirt and it’s a little big. I’m supposed to wear it the whole week and im working with kids so they prefer your Tee to be clean and not tied up to shorten or tighten it. It also has a decal on it and I do not want that to come off or become crackle looking.It also says it’s pre-shrunk but i don’t believe anything is really pre-shrunk. Please give me some good advice.