When my little nephews 5yrs, 7yrs and 9yrs are sleeping, they keep on talking. There was a time one of them started laughing right inside the sleep. How am i going to stop them? Or wil they stop it when they grow up?

Depends on what they saying in their sleep.
If they say then wanna kill you, better run.
not sure of how to stop it

Its a sleep disorder, depends on the activities of the day and food eaten b4 they sleep. If they sleeptalk daily, then u have cause for concern!

Te he he.somniloquy !
when kids eat late and play alot,they tend to continue their play and talks at dreamland.Broken sleep cycles too cause sleep talking/playing.no cause for alarm jor.I sometimes talk and play too,after a good meal,a lirrle wine and some good and fulfiling *** with hubby.Nothing spoil ! its like asking if its wrong for babies to smile in their sleep.and,Yes,they wud outgrow it.

Too much adrenalin They will get over it, they probably play too much during the day. Dont think its anything serious, just make sure thet are not over active during the day.

Think it means they had a fun day. I know that when littler ones are falling asleep they often act out everything that happened that day again as they are dozing off. I think its sweet.

bad

Nothing absurd about it.

sleeptalking is a parasomina just like teeth-grinding during sleep or  common sleepwalking.
there is nothing wrong or crazy with it, adults do talk in their sleeps as well but it is most common with children. its pretty harmless unless they scream at the top of their lungs (or give out their credit card pin code).