If you intend to have them for your children, my first recommendation would be to geld them as soon as possible. As yearlings, they are now starting to go into that hormonal "I am a stud-muffin" stage.
Also, you need to get inside their heads - horses are herd animals, and they definitely have a pecking order. You need to make yourself the top in that pecking order, and when you do so - you need to use the "Two Second Rule" because if you do anything after 2 seconds have elapsed, you are wasting your time and just confusing them.
The first thing is to give them as little opportunity to nip in the first place. Make them respect your space. Never go out and "love on them" unless you are able to control the location of their mouths at all times....this means holding their halter firmly or tying them in such a way that they cannot reach you with those powerful teeth while your scratch, pet or brush them. After a while, they should realize that their mouth is for feed, not nipping.
And if they do land a nip - in a very firm, deep and loud voice - say "QUIT" (I never use the word no - it sounds too much like whoa). At the same time, I would sometimes grab a lip or the neck and pinch HARD.
Try not slap or hit them around the head as this will make them head-shy. However, I had one colt (kept intact to be a breeding stallion) years ago that just didn't get it until I landed a roundhouse right to his nose (I grew up with 5 brothers). He never tried to bite me again.