I have been trying the 'nice' way for 3mo and it's not working. I'm kind of up against a wall and thinking of pulling out of the whole thing by saying 'if the hair does not go back to normal then they cannot come here'
Are you serious??
It's one thing if they're smarting off to you and treating you like you're crap, or if they're abusing the kid or have an abusive marriage.
But to say because the child is not groomed according to your liking they cannot visit, is incredible.
They are adults, maybe you are the one who needs to get with it and let go of your kids.
edited to add:
Yes you are entitled to your viewpoint. From experience of living with a mother in law for several years I know that it's frustrating for a parent to watch their children go through life experiences and feel like their opinion is not valued or wanted.
You are entitled to your opinion, but be wary that your opinion does not become rule. There is a differance, and while that mohawk might seem like a really friggin big deal right now, once you are not seeing your kids for a length of time, you won't care if their hair is purple, polka dot or checkerboard. You'll just be glad to have the chance to hold them.