It's a well known fact that the younger you are, the more time you spend in REM sleep (correlated with dreaming.)
Newborn babies, for example, sleep about 16 hours a day and spend half of that time in REM sleep. Fetuses in the womb during the third trimester of pregnancy have the most REM sleep of all.
As you get older, your total amount of sleep decreases and within that, the percentage of your sleep that is REM sleep decreases.
Various theories about why we dream try to explain the reason for this. They basically revolve around the idea that the younger you are the more you have to learn, the more neural connections you have to develop and the more long-term memories you have to create.
Some theories that discuss this are
The theory that dreaming is a state of "Protoconsciousness"
The Continual Activation Theory of Dreams which says that we develop our long-term memories during REM sleep.
The Reverse Learning Theory of Dreams which says that when we dream we get rid of unnecessary neural connections so the brain doesn't become overloaded. The younger you are, the more neural connections are creating, so the more neural connections you need to discard.
As you get older, the content of your dreams change because instead of just having to learn about "everything" you are involved with the stresses of day to day life and your dreams start to reflect the details of what is going on in your life at the moment and memories of specific things that have happened to you.
It's interesting that you mention breathing, because I've studied voice and had a voice coach tell me to think about a child screaming at the top of his/her lungs on a bus or train, and how (even though it is extremely annoying) incredibly loud and pure the sound is. Adults have to train themselves for many years to get their voices even close to that strength and perfection because as we grow up and become part of society, we learn to restrict our voices and our breathing.
I don't know why you want to recreate those dreams but you have to remember that children's minds and adults' minds are different. If you want to recreate something close to them, I think you will have to change your ways of thinking and feeling in waking life, possibly through yoga, meditation, taking a long vacation or even counseling to deal with things that may be causing you stress.