I can't say 100% exactly how this conversation transpired because I wasn't there. Somehow, I'm sure some of it will be lost in translation...
Isaiah had to go to a babysitter on Monday morning because he is out on spring break and his mother didn't get him to the Y in time for him to go on the field trip for that morning. He told Jamie, his sitter, that Rex's name was "shirt" and he liked it - so he was going to be a shirt, too.
That's not quite what Rex's last name is. Close, but not quite.
Isaiah Shirt... has a nice ring, don't you think?