Labrador puppy biting issue

I've a 5 month lab pup. Lovely dog but he is a fcuker for playful biting. I've a 4 year old who I keep away from him and he will bite my clothes or the 4 year olds coat and pull like hell.
It's not malicious biting just playful but he'd still mark.
I need tips on how to stop this habit. I've been told by a few people to smack him in the nose each time he does it but I really don't wanna do that or be the kinda person who smacks their dog.

I've a gsd too and when we go for walks around the farm (neither have been outside the farm) they are mess fighting a lot with the lab being the instigator 90% of the time.

Both dogs are put in their kennel at night and have access to a yard and lawn all day...though they perch at the back door of the house more often than not.