Scruffy hair

Jamie: it's a really good tiger isn't it Rosa, even the hair. (about her painting)
Rosa: what that scruffy hair, like yours? Isaac & rosa: Hahahahahahaha

My bum is a bit lopsided

"My bum is a bit lopsided" - Unrelated to anything, just Isaac's wacky thoughts.

"Where do your eyes go?" - meaning where does the back of your eyes connect to!

"I'm pleased I had an injection daddy, I like having injections." - after having his flu jab

Isaac: "Do magic paintbrushes exist daddy?"
Jamie: "No, it would be good though wouldn't it, you could paint all the toys you wanted and they'd be real!"
Isaac: "Yeah but you'd have to help me, otherwise I'd paint a scribble and it would be real."

Twinky

Rosa: ikey look at my twinky
Isaac: your twinky looks like a teeny tiny bum bum