This is when you act like a kid even if you are not. You may be old yet doing an activity that children do. It may involve doing something that is not intellectually challenging as far as age goes. For example colouring in or playing easy computer games designed for a 5 year old. You may enjoy these activities regardless of age.
Basically it means doing activities set for young children that are fun, but you are too old to do.
"You're a kidbrain." "That is so kidbrain." "We're kidbrain today."
It refers to the task being too easy for the person doing the activity. games