When u get their hair cut it will always LOOK thicker after wards becuase ur cutting the ends off.
The ends of hair grows to a point then u cut this off straight. their hair isn't actually thicker, they dont just get more hair follicles after u cut it. it just looks thicker.
It's best to cut their hair regularly for 2 reasons, they'll get used to it, and your getting rid of any unhealthy ends, hair can get split ends and snap, causing it to become shorter.
My 2 1/2 yr old got his 1st hair cut at 6 months of age and has had regular trims ever since. He is so used to sitting still in the hairdressers chair that he actually fell asleep getting his last hair cut.
Try to make the 1st few times a positive experience for them, take biscuits, lolly pops or a toy. Lolly pops worked wonders for my lil man.