Boogers from your nose taste salty and kids like the salty taste from their boogers so that's why most kids eat their boogers after picking them.
Although it's gross or at least seems gross to us adults the boogers being eaten actually boosts the kids immune system which protects their body from other sickness and diseases.
Kids will eventually outgrow the booger eating phase as they get older and I used to eat boogers myself when I was a kid although now I find it gross and will try to stop my nephew from doing it when I catch him.
The kid also needs to wash their hands more often after they pick their nose and eat their boogers but if possible just try to stop it although it's not harmful.