Help with bb adjustment

I'm new to the bmx side of things but need to get up to speed with a few things as have just bought my youngest son a bmx for Xmas.

I went for a GT as knew them from bitd and have a fondness for their retro mtbs. The model i went for was the 'slammer which seemed good value looked nice too (all important for an 8 year old).

Out of the box I had to deal with a few issues. First one was within my wheelhouse: dry and tight wheel bearings. Not ideal spending Xmas day regressing and setting up cup and cone bearings, but job is done now (why they were so poorly set up in the first place I need take take up with the shop).

The issue i haven't sorted is a really notchy bb. I have little to no experience with bmx bbs and not sure where to start so hoping someone on here can help.

When pedalling back, there is a lot of resistance and distinct notchiness that feels like the bb is too tight. The freewheel seems fine.

The crank arms are held on by pinch bolts so easy to remove, but I'm not sure how to adjust the bb.

Would there be any other cause/diagnosis for this issue before I pull it apart?

My son is keen to get out on his first ride but I don't want to cause any damage.

Appreciate this is a modern bmx, but hopefully someone on here can help.

Thanks

I assume it is a standard bmx bb and not a cartridge bb.

The concept is the same as a headset,too tight and it will bind, too loose and it will wobble. You need to loosen the lock nut and adjust the cups, you can usually do this by hand, and is as you imagine, a really fine line, then tighten lock nut down onto cup.

I the headset was dry, shouldnt be, then worth checking the bb bearings, usually in cages but older ones used to be loose bearings, get plenty of grease in there

EDIT: just re-read the post, and my reply meant to say, if the wheel bearings were dry, not headset, and the bb should be the same as the wheels you have already done, axle with bearings at each end and same way to adjust it.

Cheers, will give it a go.