I don't really think its something to be afraid of. The two of them are very sensitive and easily get hyped when doing nothing. try the following.

1. get them engaged regularly e.g watch lengthy cartoon
2. cut the girl's hair (someone rightly suggested)
3. change their environment regularly.
4. caution them

This happens to people and even dogs but the degree vary. while some can control it, some cant. The situation is that the girl gets bored easily and she unconsciously gets on her hair. The boy is hyper sensitive and restless, you need to engage him a lot and divert his energy to something positive.

try engaging the girl in a talk on something of her interest once you notice. If she responds positively, she got bored of herself probably doing nothing, if she doesn't, she is bored of her environment. Take her for a short walk.

take them both out regularly, they will see things that will catch their attention and would have less time to do those things.

its a gradual control, prevent them from getting to that state that they react.

Dnt worry, they'l outgrow the habits, I remember picking my nose clean and sending watever is found ther straight to my mouth, Ewwww gross, jst d tot of it, but now u shud Ơ̴̴͡.̮Ơ̴̴͡ how I turned out, better than many lol

Habits are funny characters, I remember back in secondary school when we used to do 'night prep' that is, general night reading in the class at night.
Some of us that used to spot 'Punk' hair style used to wonder why we kept pulling out hairs while reading. It was so bad that I will carefully single out strands one by one, pull it out and then chew it with my inscissors! Thank God 'Punk' wasn't fashionable in Uni so I went low cut, and so was the deliverance!

I think low cut will actually help the girl get over the hair pulling

I pray your son doesn't have pica. I doubt it anyway but it would give him a career and may make him famous: just ask Monsieur Mangetout.